Invaporation. 259 



Invaporation. 



By W. L. Goodwin. 



About sixty years ago, Thomas Graham made some 

 curious experiments with solutions of salts, enclosing them 

 along with water in tin canisters. The solutions increased 

 in weight by condensing water vapour from the saturated 

 atmosphere. This process he called invaporation, in contra- 

 distinction to evaporation. He showed that different salts 

 invaporate at different rates, e.g., common salt having a 

 comparatively strong power of invaporation, and sodic sul- 

 phate very weak. One result of this invaporating power 

 of saline solutions is noteworthy. The atmosphere over 

 the ocean must be drier than that over a fresh-water lake. 

 The vapour tensions of saline solutions have since been 

 determined accurately, and confirm Graham's early experi- 

 ments. The process of invaporation can be watched easily, 

 and it is certainly a most interesting case of the transfer- 

 ence of masses of matter by molecular movements. If a 

 small quantity of dry sodium chloride be put in a glass 

 tube open at one end, and sealed up in a larger tube, or in 

 a well stoppered bottle, along with a quantity of water in a 

 second small tube, the salt gradually attracts the moisture, 

 forms a solution, and at length takes to itself the whole of 

 the water, so that the second small tube, becomes quite dry. 

 The process is a slow one, requiring months to complete in 

 some cases ; but, even where the proportion of water is very 

 large, the salt is not satisfied until it has taken the whole 

 of it. 



My colleague, Professor Marshall and myself, have made 

 a series of experiments with the object of measuring the 

 relative forces with which different salts invaporate. Mole- 

 cular proportions of two salts were enclosed in the same 

 space with a certain quantity of water, for which they were 

 allowed to strive until they had divided it between them. 

 The experiments were made with pairs of chlorides, and 

 different proportions of water were used with the same 

 pair of salts so as to ascertain the effect of dilution. Some 



