1867.] Physics. 279 



coefficient of dilatation of iodide of silver is alone negative, so that 

 this salt, instead of dilating, really contracts. Eaised to a tempera- 

 ture of 40° C, the coefficient is— ? 00000139. This coefficient is 

 besides variable with the temperature and the direction in the inte- 

 rior of the crystal. In the direction in which the contraction is 

 greatest, parallel to the axis, it is the roVo part for 100° C, about 

 a tenth of the dilatation of mercury. 



M. Matteucci has experimented on • the adhesion of air to 

 metallic surfaces. He heats plates of platinum in a current of 

 oxygen or hydrogen ; then he places them, while yet hot, in a closed 

 apparatus containing one of the above-named bodies. Immediately 

 there is a diminution of volume of the gases, and a formation of 

 aqueous vapour by the combination of oxygen and hydrogen. 

 It may happen that in the case of platinum the adhesion of the 

 gases is not only to the metallic surface, but also in the interior of 

 the metal itself. Metallic platinum is generally formed by ham- 

 mering spongy platinum — a substance which has the property of 

 exciting the combination of certain gases ; hence it will not be 

 unreasonable to believe that the molecular interstices opened by 

 the heat would retain the gases, and thus cause their combination 

 under the influence of other bodies in contact with them. 



Mr. Crookes has drawn attention to a curious result of the very 

 severe frost experienced at the commencement of this year. Under 

 the combined influence of cold and vibratory motion, large masses 

 of glycerin were noticed to have assumed the solid crystalline 

 state. About five tons of glycerin, in casks of eight cwt. each, 

 were recently imported from Germany, by the firm of Burgoyne, 

 Burbidges, and Squire. When they left the factory the contents were 

 in their usual state of viscid fluidity ; but on arriving in London, 

 they were found to have solidified to a solid mass of crystals, so 

 hard that it required a hammer and chisel to break it up. 



A large block of this solid glycerin, weighing several hundred- 

 weight, suspended in a somewhat warm room, took two or three 

 days to liquify ; and a thermometer inserted in the fusing mass 

 indicated a constant temperature of 45° F. In small quantities, 

 the crystals rapidly fuse when the bottle containing them is placed 

 in warm water. In quantity the solid glycerin looks very like a 

 mass of sugar-candy. The isolated crystals are sometims as large 

 as a small pea ; they are brilliant, and highly refracting ; when 

 rubbed between the fingers they are very hard, and they grate 

 between the teeth. Their form appears to be octahedral. 



The crystals, separated as much as possible from the mother 

 liquor, and then fused by heat, form a clear and nearly colourless 

 liquid, slightly more viscid than usual, which possesses all the 

 physical and chemical properties of pure glycerin. 



