342 Messrs. C. Tomlinson and G. Yan der Mensbrugghe [May 16, 



is above the mean, indicating dissociation. The specific heat of these 

 mixtures does not, however, show any corresponding alterations. 



The author finally shows that if these propositions should prove to be 

 an exposition of a general law, they will enable us to calculate the specific 

 heat of one constituent of a mixture or solution, if we know the specific 

 heat of the other constituent, the specific heat of the mixture, and the 

 heat evolved or absorbed at two different temperatures between which 

 these specific heats have been estimated. It may thus be possible to com- 

 pare the specific heat of liquid and solid substances when both are placed 

 under similar conditions, and relations may be discovered which at present 

 are masked in consequence of the difference in the physical conditions of 

 the substances to be compared. 



II. "'On Supersaturated Saline Solutions. Part III. — On a relation 

 between the Surface-tension of Liquids and the Supersaturation 

 of Saline Solutions." By Charles Tgmltnson, F.R.S., and 

 G. Van der Mensbrugghe. Received April 6, 1872. 



It was stated by one of us in Part II. * that when a drop of a liquid is 

 deposited on the surface of a supersaturated saline solution, it will do one 

 of three things :— (1) mingle with the solution without any nuclear action ; 

 (2) spread out into a film with powerful nuclear action ; or (3) assume the 

 form of a lens, without any separation of salt. It was further stated that 

 when a liquid forms a film or a lens, it does so according to the general 

 proposition, that if a drop of a liquid B, with the surface-tension b, be 

 placed on the surface of another liquid A, with the surface-tension a, the 

 drop will spread into a film, if a > b + c (c being the tension of the common 

 surface of the liquids A and B) ; but if, on the contrary, a= <b-\-c, the 

 drop will remain in the form of a lens. Hence if B spread on A, A will not 

 spread on the surface of B. When the liquids A and B mingle in all pro- 

 portions, chas no value. The spreading of the drop may also be interfered 

 with by the superficial viscosity of the solution, or the greater or less 

 difficulty in displacing the superficial molecules. 



It was also stated that if a greasy smear be made upon the clean interior 

 surface of a flask above the solution, and the flask be inclined so as to 

 bring a portion of the solution against such smear, the liquid does one of 

 two things : — (1) it breaks up into well-defined globules, which roll over 

 the smear without loss of tension, in which case the smear has no nuclear 

 action ; or (2) as soon as the solution reaches the smear its edge flattens 

 and becomes ragged, in which case the smear is nuclear and the salt 

 separates. 



, A glass rod drawn through the hand becomes covered with a smear or 

 film ; or the same roc 1 , by exposure to the air, contracts a film by the 



* Philosophical Transactions for 1871, p. 52. 



