898 



aperture or mouth of the phial 1*25 inch in diameter. The phial was 

 filled up with the solution to be diffused till it reached the point of 

 a pin dipping exactly 0*5 inch into the mouth of the bottle. This 

 being the solution cell or bottle, and the external jar the " water- 

 jar," the pair together form a " diffusion cell." The diffusion was 

 stopped, generally after seven or eight days, by closing the mouth 

 of the phial with a plate of glass, and then raising it out of the 

 water-jar. The quantity of salt which had found its way into the 

 water-jar — the diffusion product as it was called — was then deter- 

 mined by evaporating to dryness. 



The characters of liquid diffusion were first examined in detail 

 with reference to common salt. 



It was found, first, that with solutions containing 1, 2, 3 and 4? 

 per cent, of salt, the quantities Vv^hich diffused out of the phials into 

 the water of the jars, and were obtained by evaporating the latter, 

 in a constant period of eight days, were as nearly in proportion to 

 these numbers, as 1, 1*99, 3*01 and 4-00; and that in repetitions of 

 the experiments, the results did not vary more than l-40th part. 

 The proportion of salt which diffused out in such experiments 

 amounted to about l-8th of the whole. 



Secondly, that the proportion of salt diffused increases with the 

 temperature ; an elevation of 80" Fahr. doubling the quantity of 

 chloride of sodium diffused in the same time. 



The diffusibility of a variety of substances was next compared, 

 a solution of 20 parts of the substance in 100 water being always 

 used. Some of the results were as follows, the quantities diffused 

 being expressed in grains : chloride of sodium 58'68, sulphate of 

 magnesia 27"4'2, sulphate of water 69*32, crystallized cane-sugar 

 26*74-, starch-sugar 26*94, gum-arabic 13-24, albumen 3'OS. The 

 low diffusibility of albumen is very remarkable, and the value of this 

 property in retaining the serous fluids within the blood-vessels at 

 once suggests itself. It was further observed, that common salt, 

 sugar and urea, added to the albumen under diffusion, diffused away 

 from the latter as readily as from their aqueous solutions. Urea 

 itself^s as highly diffusible as chloride of sodium. 



In comparing the diffusion of salts dissolved in 10 times their 

 weight of water, it was found that isomorphous compounds generally 

 had an equal diffusibility, chloride of potassium corresponding with 

 chloride of ammonium, nitrate of potash with nitrate of ammonia, 

 and sulphate of magnesia with sulphate of zinc. The most remark- 

 able circumstance is that these pairs are " equi-diffusive," not for 

 chemically equivalent quantities, but for equal weights simply. The 

 acids differed greatly in diffusibility, nitric acid being nearly four 

 times more diffusive than phosphoric aeid ; but these substances 

 also fell into groups, nitric and hydrochloric acids appearing to be 

 equally diffusive ; so also acetic and sulphuric acids. Soluble sub- 

 salts and the ammoniated salts of the metals present a surprisingly 

 low diffusibility ; the quantities diffused, under similar circumstances, 

 of the three salts, sulphate of ammonia, sulphate of copper, and the 

 blue ammonio-sulphate of copper being very nearly as 8, 4 and 1. 



