pare the result with the wave-lengths of the notes of the scale reduced to the same ratio. Here are the limits of wave-lengths of the different colours of the spectrum as most carefully determined by Prof. Listing.* In the third column the writer has added the mean wave-length of each colour, and in the fourth column the ratio of one colour to another, taking the mean wave-length of red as 100.

TABLE OF WAVE-LENGTHS OF COLOURS IN THE SPECTRUM.

WAVE-LENGTHS: IN MILLIONTHS OF A MILLIMETRE.

Here next is a table giving the middle notes of the scale, their wave-lengths, and their reduction to a common ratio, taking C as 100.

TABLE OF WAVE-LENGTHS OF NOTES OF SCALE.

Putting together the two ratios, the following remarkable correspondence at once comes out:

RATIO OF WAVE-LENGTHS OF NOTES COMPARED TO RATIO OF WAVE-LENGTHS

Assuming the note C to correspond to the colour red, then we find D exactly corresponds to orange, E to yellow, and F to green. Blue and indigo, being difficult to localize, or even distinguish in the spectrum, they are put together: their mean exactly corresponds to the note G. Violet would then exactly correspond to

* Poggendorff's 'Annalen,' vol. cxxxi., p. 564.

the ratio given by the note A. The colours having now ceased, the ideal position of B and the upper C in the spectrum are calculated from the musical ratios. This coincidence, as unexpected as it is perfect, is represented in the two upper figures on the Plate.*

Had space permitted, we should have ventured to trace out to some extent this common harmony of colour and sound. All we can do is to point out a few suggestions that occur at once.

Every one knows that the juxtaposition of two colours nearly alike is bad, and it is well known that two adjacent notes of the scale sounded together produce discord. Selecting and sounding together two different notes we may produce either discord or harmony; so with the juxtaposition of certain colours, either pleasurable or painful effects are produced. Thus-the notes D and E, together, are bad; so are orange and yellow when contrasted. C and G harmonize perfectly, so do red and blue. C and F is an excellent interval, so is the combination of red and green. Now, on referring to the Plate, it will be seen that the foregoing notes exactly underlie those very colours that we have named with them.

But, further, it is possible to obtain a real optical expression of the musical intervals. By reflecting a beam of light from one vibrating tuning fork to another placed at right angles, curves of light are obtained, which vary according to the combination of forks we select. The most perfect harmony, viz. two notes in unison, gives the simplest curve-a circle. The next most harmonious interval, an octave and its fundamental note, gives the figure of 8; the next, the interval of a fifth, gives a more complete figure, and so on. The complexity augmenting as the concord lessens. Some of these curves are shown on the lower figure in the Plate. By the side of each curve is put the musical notes from which it was derived, and for the sake of comparison the colours which would correspond to each interval are also brought down. It will be seen that harmony runs throughout.

A musical chord thus becomes both a representative picture, and an acoustic painting, whilst the musical scale is literally a rainbow of sound. It is hardly too much to say that we might possibly translate into a musical melody a sunset, a flower, or a painting by a Rubens or a Raphael.‡

But here let us check our imagination. We have throughout the foregoing article endeavoured to avoid overstating the analogy. Let us now be careful lest we become victims of the "idola tribus," lest we strive to impose on nature a greater degree of simplicity than her facts will justify.

There will be noticed over the spectrum on the Plate a scale of actual wavelengths, by which the remarkable but natural crowding together of the colours at the red end is well seen.

+ First accomplished by M. Lissajous.

On this subject an able article by Mr. C. Seth Smith, recently appeared in

theBuilder.'

II. ON THE PRINCIPLES AND METHODS OF
SEWAGE IRRIGATION.

EWAGE utilization is perhaps one of the most hotly-debated subjects of the day, and frequent references to it have from time to time appeared in the pages of this Journal. These will be found repeatedly in the Chronicles of Science (Agriculture), and in two articles, entitled respectively "Sewage and Sewerage," and "On the application of Sewage to the Soil," wherein the progress made in the development of works for sewage irrigation purposes has been recorded. Our present object is to give a brief account of the best means for carrying out irrigation works for the disposal of town sewage, and of the laying out of lands preparatory to the application of sewage, so far as they can be deduced from the results of past experiments, and from works hitherto constructed and brought into operation in different parts of the United Kingdom. We shall, however, preface our remarks on the above-named subjects by a reference to one or two points in connection with them, with the view to show that the present movement in favour of utilizing our town sewage is but the revival of a practice of great antiquity, which, owing to numerous causes, has, for many centuries, been abandoned and perhaps forgotten.

The recognized power of earth to act as a disinfectant may first be traced to the Mosaic lawgiver, but it is not improbable that it was applied to that purpose before the departure of the Israelites from Egypt, and that the injunction for it to be so used whilst they were on their wanderings was but a law for the observance of a then well-known sanitary precaution. The filth of Jerusalem was, it is recorded, at one time burnt in an oven in the valley of Hinnom, which also served for human sacrifices, and was called "tophet," from "toph," a drum, used on such occasions to drown the cries of the victims. At a later period, however, when the Mosaic religion. was restored, the Temple purified and rebuilt, and the country began to prosper under the protectorate of powerful neighbouring nations, large sewers and aqueducts were constructed, which still exist, owing to the fact of their being cut in the solid rock upon which the city was built. Eusebius, who was a native of that country, 'Quarterly Journal of Science,' 1866, p. 180. + Ibid., 1867, p 357 VOL. VII.

C

and died about the year 340, mentions Timocrates, the surveyor of Syria, by whom the city was throughout provided with water. The water used for flooding the court of the Temple, to wash away the offal and blood of the sacrifices, drained into a pit, now called "The Fountain of the Virgin;" from whence, after mingling with the town sewage, it was conducted to a second one, now called the Pool of Siloam-but which, it is thought, is not the one formerly known by that name—and thence to the king's garden, for purposes of irrigation. These pits served, no doubt, as settling-tanks to collect the solid matter; and thus, in their general arrangement, we can perhaps trace the earliest recorded attempts at utilizing sewage, and one which, so far as our information goes, does not appear to have differed materially from the most approved practice of the present day.

The difficulty in getting rid of night-soil and refuse in large towns by any other method has necessitated the adoption of sewers and water-carriage for that purpose, and with our present knowledge on the subject it does not appear probable that sewers will ever be superseded. "It matters not," remarked Mr. Bailey Denton in a recent letter to 'The Times'* newspaper, "whether the earth-closet system of excretal sewerage gains ground in places where advantageous circumstances suggest its adoption, sewers must exist in every place where habitations are congregated together, whether it be a city or a village, for the discharge of liquid refuse from the chamber, the bath, and the kitchen, independently of the excrements of the closet, which form in reality but a small proportion of the entire refuse of the dwelling." The introduction of sewers in places has, as might naturally be expected, led to their being used also as drains, and the result appears to have been satisfactory, although the reports by the Medical Officer of the Privy Council tend to show "that where a system of separating the sewage of dwellings from the water of the soil on which they stand has been adopted, and the sewerage and drainage can be discharged by different channels, the maximum of success may be achieved." On the other hand, Mr. Denton states that "wherever sewerage and drainage which have different sanitary effects, and ought to be distinct operations-have not been carried out together, intentionally or accidentally, the operations have failed, more or less, in the purposes for which they were designed."

The mixture of water with sewage is looked upon by some agriculturists as a great drawback to its application. Apart, however, from water-carriage being the cheapest, as well as the most convenient form of removing the sewage of towns, it is of value in distributing it, and enables the operation of spreading solid manure over the face of the earth, which must otherwise take place, to be * Times,' October 28, 1869.

·

dispensed with. By the process of irrigation, too, fertilizing matter is distributed over the land with uniformity, and it is presented to the plant in such a state that it is at once ready to be assimilated, that is, it is at once food for the plant; the plant grows more rapidly, the period of growth is greatly shortened, and, consequently, we get a greater number of crops in a given period, under the irrigation system, than could possibly be obtained under any process of dry manuring. The quantity of water mixed with the solid matters in the ordinary sewage of towns is very great, and it has been estimated that, including rainfall, 350 parts of water are employed in removing one part of excrement; thus the sewage is delivered to the land in a very diluted state, but, as has been proved by results, by no means too weak for useful application. The strength of pure sewage would be far too much for vegetation, and, instead of improving it, would tend utterly to destroy it; but thus diluted it is reduced to a state in which it appears to be most readily absorbed by the earth, and thence taken up by plants as it is required for their nourishment.

In order to meet the requirements of local circumstances, where land is not available for purposes of irrigation, attempts have from time to time been made to separate the solid particles from the fluid, the former being made into a species of artificial manure, whilst the latter is allowed to pass away into the most convenient channel for its escape. The value of the manurial ingredients held in solution, being to that contained in the solid portions as six to one, the great fertilizer ammonia also being afloat in the liquid portion, it is not to be wondered at that these experiments have invariably failed, and the works erected for carrying out the different processes have, almost without exception, been abandoned. After filtration, the general plan has been to mix the solid residuum with dry rubbish, town ashes, charcoal, or other bases for forming a solid substance; the unwillingness, however, of farmers to purchase this manure at a remunerative price to the manufacturers, and often their refusal to pay for it at all, necessarily led to the early closing of all works constructed for the purpose of its manufacture.

In order to counteract the loss of valuable manurial ingredients. which, under the above processes, passed off with the liquid portions of the sewage, recourse was next had to the use of chemical reagents with the view of causing a precipitation of those fertilizing ingredients which are held in solution, and for this purpose use has been made of lime, sulphate of alumina, soluble phosphate of magnesia, perchloride of iron, &c.; but as none of these have been successful in causing a precipitation of ammonia, or any other manuring substance, it is needless to enter here into any further details regarding these experiments. Suffice it to state that no