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XXXVIII. Supplementary Observations on the Diffusion of Liquids. 
By Thomas Graham, F.R.S., F.C.S. 
Received May 2, — Read June 20, 1850. 
The experiments of my former paper furnished strong grounds for believing that 
isomorphous salts possess a similar diffiisibility. All the salts of potash and ammonia, 
which were compared, appeared to be equi-diffusive ; so also were the salts of certain 
magnesian bases. A single preliminary observation on the nitrates of lead and baryta, 
however, opposed the general conclusion, and demanded further inquiry. It is scarcely 
necessary to say that any new means of recognizing the existence of the isomorphous 
relation between different substances, must prove highly valuable. Let us inquire 
therefore how far liquid diffusion is available for that purpose. 
The salts were still diffused from weak solutions, that is from solutions containing 
from 1 to 8 per cent, of salt ; but now a measure of the solution, equal to 100 grs. of 
water, was made to contain 1 grain of the salt, to form what is called the 1 per cent, 
solution ; instead of 1 grain of salt being added to 100 grs. of water, as before, without 
reference to the condensation which generally occurs. The quantities 1, 2, 4 and 8 per 
cent, thus indicate the parts of salt present in a constant volume of liquid, — as 10, 
20, 40 and 80 grs. of the salt in 1000 water grain-measures of the solution. The 
same phials for the solution and jars for the external water-atmosphere continued to 
be used, and the manipulations were similar. It is believed, however, that the tem- 
perature of the liquids was maintained more uniform in the new experiments than 
the old, partly by the better regulation of the temperature of the apartment, and 
partly by placing the jars close together upon a table with upright ledges, and cover- 
ing the whole over with sheets of paper during the continuance of an experiment. 
The mass of fluid in 80 or 100 jars, which were employed at once and placed together, 
made the small oscillations of temperature, which might still occur, slow and less 
injurious. 
The investigation is also extended to several new substances, such as hydrocyanic 
acid, acetic acid, sulphurous acid, alcohol, ammonia and salts of organic bases, without 
reference to isomorphous relations. It is very necessary to have data which are 
minute and accurate respecting the diffusion of a considerable variety of substances. 
This it is my present object to endeavour to supply, leaving speculative deductions in 
general respecting the nature and laws of liquid diffusion for a future occasion. 
The density of all the solutions was observed at a constant temperature, namely, 
60° Fahr. 
