410 Professor W. Chandler Boherts- Austen [March 26, 



selves in bottles become coherent, and if the coherence in these and 

 other chemical compounds is but weak, it is simply because the points 

 of contact between the i^articles of powder are but few. If, on the 

 other hand, metallic or other powder be submitted to strong com- 

 pression, the spaces between the fragments become filled with the 

 debris of the crushed particles, and a solid block is the result. 

 Finally, it may be urged that this union of powders of solid metals 

 under the influence of pressure, that is to say, the close approximation 

 of the particles, can be compared to the liquefaction of gases by 

 pressure. At the first view this comparison may appear rash or 

 strained, but it is nothing if we accept the views of Clausius on the 

 nature of gases and liquids. In a gas the molecules are free, but if 

 by pressure at a suitable temperature the molecules are brought 

 within the limits of their mutual attraction, the gas may be liquefied, 

 and under suitable thermal conditions solidified. The mechanical 

 pulverisation of a metal merely detaches groups of molecules from 

 other groups, because the mechanical treatment is imperfect, but the 

 analogy between the solid and a gas has, in a sense, been established ; 

 filing has coarsely gasified the mass, but pressure will solidify it as 

 you have already seen. 



It is possible that in some of these metallic blocks, the particles 

 are not actually united by the pressure, which may, nevertheless, 

 develop the kind of "mutual attraction" contemplated by Sir W. 

 Thomson as existing between two pieces of matter at distances of less 

 than 10 micro-millimetres. 



There are two other properties which solid metals possess, in 

 common with certain fluids, to which I must briefly allude. The 

 first is the power of dissolving gas, which metals in the solid colloid 

 condition possess. I will not offer any experimental illustration on 

 this point, because the work of Graham has been fully dealt with in 

 this theatre by Dr. Odling, and I have, in a course of lectures recently 

 delivered here, shown that just as solid palladium occludes hydrogen, 

 so the alloy of rhodium and lead occludes oxide of nitrogen, which it 

 gives up with explosive violence on heating in vacuo, suggesting an 

 analogy with fluid nitro-glycerine. The last property I have to 

 submit to you, is the power which certain solid metals possess of 

 taking up fluids, sometimes with a rapidity which suggests the 

 miscibility of ordinary fluid substances. In reference to this I have 

 found an interesting paper published so long ago as 1713, by the 

 Dutch chemist Homberg,* " On Substances which penetrate and 

 which pass through Metals without melting them." He enumerates 

 several substances which will pass through the pores of metals with- 

 out disturbing the particles, and he points out that mercury penetrates 

 metals without destroying them. Few of us are, I think, familiar 

 with the rapidity with which mercury will pass through tin. Here 

 is a bar, one inch wide and half inch thick ; if a little mercury be 



* ' Mem. de I'Acad. Royalc dcs Sciences,' 1713 [vol. for 1739, p. 30G]. 



