parenthesis, contains the point at issue. Mallet's method of settling the dispute was to pour into an iron vessel liquid castiron, and the contents of the vessel being known, data were thus obtained by which the density of the liquid and solid were obtained. All necessary corrections were made, and the results obtained were that the cold cast-iron had a sp. gr. of 7·170, the liquid 6-650, and that at the solidifying point there was no apparent expansion. Now, it is so well known by practical men that cast-iron does expand, that it seemed of some importance to find why this was not shown by Mallet's method. I had tried for me a number of experiments on this subject which I have communicated to the Royal Society of Edinburgh,* and of which the following is a summary:-In pouring cast-iron, at what Mr. Mallet terms a yellow heat-into an iron mould, unless that mould be very thick, it yields easily to the mass when solidifying-hence the volume of the vessel cannot accurately be determined, as the vessel yields not equally but at irregular points. The only method which can give satisfactory results is that of immersing solid metal of the same composition in a bath of the liquid. Mr. Mallet objects to this, as he says that solids float on liquids which are specifically lighter, but this never takes place when the solids have been immersed for some time below the surface.

Mr. Mallet, to account for solid iron floating upon the liquid, calls in a new force-" repellant force”—and says "that its amount is, cæteris paribus, dependent upon the relation which subsists between the volume and effective' surface of the floating piece." I think that the experiments I am about to describe will dispel all uncertainty about the matter. Spheres of iron were cast from 3 to 6 inches in diameter, and their solidity determined by breaking one of a pair which weighed exactly the same. There were no hollows found, so that we had here a solid sphere of iron. A bath of the same metal was taken, about 4 feet wide by 2 feet deep, and one of the spheres dropped in. It sank immediately, and on raking the pot with a rod, it could be felt at the bottom. In about 50 seconds, however, it came to the surface, and had a temperature of dull red. As its temperature increased it floated higher and higher till it began to melt, when its height remained constant, and then, of course, diminished as the sphere melted away. Another experiment was done with the same results, and the value

* Read at the Meeting on 15th December, 1879.

was found to be constant. Some spheres were then floated and fished out when at their maximum temperature, and the "tide mark" left by the contact with the liquid surface showed the relation between the volume or densities of liquid and solid metal.*

Thus it is plain that cast-iron does expand upon freezing. The results obtained are stated in the paper, and were found to be that on an average cast-iron expands 562% of its volume on solidification. This might seem sufficient to set the matter at rest, but I have attacked the problem in another way, which proves the above. It was noticed that the temperature at which the cast-iron spheres floated level with the surface of a bath of similar metal was a dull red heat, and a large number of experiments were made by heating cast-iron spheres to various temperatures and plunging them in the molten metal. By this method the other result was confirmed-viz., that the iron just floats, touching the surface of the liquid at a red heat, estimated to be at 750° to 800°. Now, it remained to determine the expansion up to that temperature, and this was done by observing its linear expansion in the usual apparatus-where the expansion of a bar of metal deflects a telescope-and calculating the cubical dilation. A bar of cast-iron was placed on such an apparatus and surrounded by live charcoal, and when its temperature was raised (as nearly as could be judged, the same temperature as that at which the iron floated) its expansion was read off. Four experiments gave as follows:-000055,

000070, 00005, and 000065, giving a mean of 000060 per degree centigrade. The density of the iron I experimented with was 7.214, which decreased to 6.883 at about 800°, and this was the density of the liquid; but as a sphere floated with 5.62% of its volume above the liquid, the density of the solid at its fusing point must have been reduced to at least 6.535. I say at least, because as the experiments had to be done with iron considerably above its fusing point, in order to prevent its freezing while the experiments were being conducted, the result is under the truth. Now, Mr. Mallet's iron had a density of 7·170 solid and 6.650 liquid, and supposing that the co-efficient of expansion did not increase (which is very unlikely), his own would be reduced to 6.56 at 1500°, which shows that the iron would have floated. A curve appended illustrates the expansion.

Mr. Mallet admits that the metal bismuth does expand on freezing, and this I find to be the case, although, as far as I can determine, to a slightly less extent than cast-iron, the estimates I have made giving 4.8% as the amount of expansion on solidifying. Mr. Mallet instances lead as a proof that iron does not float, but it has long been well known that solid lead does not float, as it has also been known that iron does. On page 216 Mr. Mallet says, in regard to the causes of flotation: "B. A repulsive force of some and tending to repel the surfaces in contact with the hot and cold, metals," and goes on to explain this, but one experiment like the following must at once dispel all such theories :-A sphere of solid cast-iron, 6 inches in diameter, was placed in molten iron, and at once sunk, but after about 40 seconds floated again. When it reached its melting point it was washed with molten metal, and every part of it thoroughly "wet" with it, as was seen by it forming a meniscus at the line of flotation, no matter what side was turned up, and this was continued persistently till the whole sphere was melted. Now Mr. Mallet might just as well try to explain the floating of ice on water by a "repellant force" explain this floating of iron. Why, also, if iron floats by repellant force, does not copper do the same?

Further, from a letter published in the Proceedings of the Philosophical Society of Glasgow, it appears certain the slags, and even molten granite, show the same phenomenon-the solid floating on the liquid; and from experiments I have done at blast-furnaces on the Clyde, I have invariably found the same result, but have had as yet no opportunity of making measurements.

6.50

Meanwhile, Messrs. Nasmyth and Carpenter's theory, that there are two causes-expansion and contraction-for the present configuration of planetary and lunar surfaces, seems to be probable, Mr. Mallet's refutation of that theory, by his experiments upon iron, being unsound. As it appears unlikely that the telescope will ever reveal to us the action of the first of those volcanic forces (those planets which are still hot, Jupiter and Saturn, being enshrouded in clouds of vapour which will form their seas), we must look to the laboratory as likely to yield us the true explanation of the configuration of the surface of a mass cooled from fusion. In relation to the annexed curve, it appears that the rate of expansion does not increase very much at high temperatures; but, as I have explained, this is in part owing to the measurement of the expansion being low. Mr. Mallet's numbers cannot be taken as very accurate, owing to the plasticity of the containing vessel at a "yellow" heat.

X.-An Experimental Investigation into the Trap and Water-closet System, and the relation of the same to Sewage Products, gaseous and other. By NEIL CARMICHAEL, M.D., C.M., Fellow of the Faculty of Physicians and Surgeons, Glasgow, one of the Vice-Presidents of the Biological Section of the Philosophical Society, &c.

[Read before the Society, February 18th, 1880.]

"A GOOD water-closet is the only appliance fit to be used within a house, for by it all matters are at once conveyed away, and cease to have the power of producing evil so far as our houses are concerned." Such is the opinion expressed by one of our ablest sanitary engineers, Mr. Baldwin Latham.* This opinion is, however, not acquiesced in by all sanitarians, some asserting that the water-closet system is essentially erroneous in principle, and a serious source of danger. Opinions have been freely expressed on both sides of the question without leading to any very definite conclusions. As a matter of this sort could only be decided on careful consideration of evidence obtained by accurate experiment, I set myself to do what was in my power to settle the points at issue, as matters of fact. But before detailing to you the methods and results of this research, permit me to refer, as briefly as possible, to some points in relation to the water-closet system which it will be necessary clearly to comprehend before the objects and the value of the results obtained can be duly appreciated.

The form of water-closet most generally used, and the one almost exclusively used in Glasgow, is known as the pan closet. It consists of an earthenware basin, a copper cup or pan placed under the latter and containing water so as to close the lower

*

Sanitary Engineering, by Baldwin Latham, p. 506. Second Ed. 1878.