Richards — Transition Temperature of Sodic Sulphate. 203 



Again, the "melting" absorbs enough heat to insure the ren- 

 dering " latent" of any heat evolved by the mechanical process of 

 gentle agitation, or of any heat taken in from the environment. 

 The semi-opaque nature of the mixture removes any serious 

 danger from radiant heat. 



Since the volume scarcely changes with the transition, rea- 

 sonable alterations in pressure do not cause any essential change 

 in the temperature. This fact may be of importance if one has 

 occasion to use very deep layers of the mixture. 



The number of possible hydrates is smaller in this case than 

 in many, hence the change is a sharp one. The salt 

 ]N"a 2 S0 4 .7H 2 is so soluble that it cannot exist in the presence 

 of either of the other solid phases.* 



Last, but not least, Glauber's salt is extremely inexpen- 

 sive, and at hand in every chemical and physical laboratory. 

 Oddly enough, the melting point of the anhydrous salt, 865°, 

 is one of the most satisfactory standards among high nonvar- 

 iant temperatures. 



It is needless to state that the usual precautions necessary to 

 obtain accurate results were taken in the thermometric work 

 which follows, as well as in the chemical preparation. Several 

 thermometers were used in the course of the experiments; they 

 will be described as the account progresses. For the first 

 determinations, which were wholly of a relative nature, a Beck- 

 mann thermometer of the first quality, made by Goetze of 

 Leipzig, was chosen. This instrument was graduated to hun- 

 dredths of a degree, and could be easily read to within a thou- 

 sandth. Because of the thinness of its bulb, this thermometer 

 was especially sensitive to pressure ; hence it was guarded 

 carefully during its use. It was always read in a vertical posi- 

 tion, as were all the other thermometers. Since the temperature 

 of the room remained essentially constant during the prelimi- 

 nary experiments, the barometric pressure did not change by 

 important amounts, and the height of the mixture around the 

 bulb was always about the same, the readings are directly 

 comparable with one another without correction. 



First Problem — The effect of the Temperature of the Environment. 



At first sodic sulphate prepared by crystallizing twice in a 

 porcelain dish the ordinary " purissimum " material of com- 

 merce was used. This preparation was coarsely powdered and 



*The non-existence of the substance Na 2 S0 4 .HoO may now be considered 

 proved. Even if it really exists, however, the facts recorded below remain ; one 

 must merely read " monohydrated " instead of " anhydrous, 1 ' and the theory will 

 be unchanged. See de Coppet, Berichte d. d. Ch. Gesell., xii, 248; also, Lescoeur, 

 Ann. Chim. Phys. [6], xxi, 526(1890). 



