122 SCIENCE PROGRESS. 



the salt diffused in it with the rate at which salt dif- 

 fused in pure water. He found that the diffusion in the 

 jelly took place almost, if not quite, as fast as in water 

 itself. The composition of the jelly was 2 per cent, gelose 

 and 98 per cent, water, so that, as far as actual substance 

 was concerned, the salt had to meet practically the resistance 

 of water alone in both cases, and the experiment showed 

 that the mere change in apparent condition of the whole 

 mass had little or no influence on the rate of diffusion. 

 Subsequent experiments have served to confirm Graham's 

 results. 



When we pass to solids proper we find that instances 

 are not wanting of what is apparently diffusion within them. 

 Van't Hoff in his fundamental paper on solid solutions gives 

 numerous examples. In the preparation of steel by the cem- 

 entation process bars of wrought iron are packed in charcoal 

 and subjected to a red heat for several days. The charcoal 

 gradually penetrates the iron and converts it into steel. It 

 matters little for our purpose what the particular form is that 

 the carbon assumes during its passage through the iron — in 

 some fashion or other it reaches the centre of the dense bar. 

 The distribution of the carbon, too, if the operation is inter- 

 rupted before uniformity has been attained, is precisely what 

 would be expected if the phenomenon were one of real diffu- 

 sion ; and the influence of time is the same in both processes. 

 Not only has carbon been observed to pass through iron, 

 but it has even been proved to travel slowly through por- 

 celain, when porcelain crucibles have been heated in a bed of 

 graphite. 



When a metal such as copper is deposited galvanically 

 on another metal, it penetrates beyond the surface of the 

 latter into its substance, and zinc objects which have been 

 lightly coppered are, even when protected by a coating of 

 varnish, occasionally observed to become white again 

 owing to the gradual mixing of the two metals near the 

 surface. 



Professor Spring, of Liege, who has devoted special 

 attention to the chemical behaviour of solids under high 

 pressure, has supplied some interesting instances of pheno- 



