256 PLYMOUTH INSTITUTION. 



course, by opening the intervals more or less, any required pro- 

 portion of the light could be cut off, and measured by compari- 

 son with a given standard : this would apply to lights of any 

 degree of intensity, or of any temperature, but still seemed sub- 

 ject to difficulty in case of lights much different in colour. 



Professor Graham's researches on the diffusion of gases, formed 

 the second subject of the report. It was shewn, that a tube 

 about a foot long, the upper end plugged with plaster of Paris, 

 being filled with hydrogen gas, the gas made its way through the 

 plug, so that the water rose quickly in the tube, four or five in- 

 ches above its level in the trough, the contrary effect resulted 

 with carbonic acid, but the tube broke before it was shewn. 

 The principle was stated to apply to all gases ; those which were 

 lighter than air, escaping faster than the atmospheric air took 

 their place, and the water consequently rising in the tube, above 

 the level in the trough : those which were heavier than air, pass- 

 ing off slower than the air entered, and the water falling lower in 

 the tube than its level without, and the rate of diffusion proved 

 to be, for each gas, inversely proportional to the square root of 

 its density. 



Isomerism, or identity in composition of bodies differing in 

 physical and chemical properties, was the next subject. 



The nature of definite proportions was illustrated, by mix- 

 ing a solution of 60 grains of potass, with a solution of 83 

 grains of tartaric acid ; the result being a soluble neutral^alt, and 

 the liquor remaining clear. Another equal portion of tartaric 

 acid being then added, bitartrate of potass, or cream of tartar 

 resulted; which being much less soluble, immediately made the 

 liquid dense and fell in a copious precipitate. The same law 

 was shown to hold good, in double decomposition, of acetate of 

 lead by sulphate of copper : and from these and other illustrations, 

 was deduced the atomic theory. 



It was then shewn, that many substances of considerably 

 different properties, were not only composed of the same ingre- 

 dients ; but also in the same atomic proportions ; thus shewing 

 that remarkable differences may be produced by mere difference 

 of arrangement of the same atoms ; and it is yet quite uncertain 

 to what extent this may go. 



The only remaining subject of the report was isomorphism, a 

 sort of counterpart of the last ; for as that related to substances 

 differing in properties, but identical in composition, so this be- 

 longs to bodies identical in crystalline form, but different in 



