20 Mr. F. Guthrie on Salt Solutions and Attached Water-. 



truth to assert that the number of known bodies may soon 

 be doubled. 



§ 38. Geological. — The behaviour of mixtures of salts will 

 again offer a new chapter for study ; and I suppose we may 

 expect that much light will be thus thrown upon some of the 

 most obscure geological questions. For though we have been 

 considering above cryohydrates (that is, compounds of water 

 solidifying below the freezing-point of water), there can be no 

 discontinuity separating the medium water with its peculiar 

 temperature of solidification from other media of very different 

 melting-points. We know already, indeed, very many instances 

 in which the mixture of two bodies has a lower melting-point 

 than either of its constituents. What must happen, then, if a 

 mass of molten rock, such as a silicate, is saturated at a high 

 temperature with another silicate ? When the mixture cools, 

 the second may separate out in the solid form, perhaps as 

 quartz, perhaps as felspar, or what not. Anon, at a certain 

 lower temperature, solidification takes place between the medium 

 and the dissolved rock in definite proportion — definite, though 

 perhaps not necessarily in chemical ratio, but presenting that 

 mineralogical ratio which is so striking, and which has not 

 hitherto been satisfactorily explained. 



§ 39. Constant temperatures. — Perhaps one of the most in- 

 teresting aspects of the experimental results is the establish- 

 ment of fixed temperatures below zero. With the exception of 

 the melting-points of a few organic bodies such as benzol, and 

 the boiling-points of a few liquids such as liquid ammonia, sul- 

 phurous acid, and carbonic acid, and the rather ill-defined tem- 

 peratures to be got by various freezing-mixtures, there are no 

 means in the hands of physicists for obtaining and maintaining 

 with certainty and ease a fixed temperature below 0° C. Now, 

 if we surround a body with one of the solid cryohydrates de- 

 scribed above, the body is kept at a corresponding tempera- 

 ture as long as any of the cryohydrate remains solid, and this 

 with as much certainty as the temperature 0° C. can be main- 

 tained by melting ice, We thus command temperatures between 

 — 23° and 0° C. with the greatest precision. 



§ 40. Invitation to others. — I need scarcely point out that the 

 field of inquiry which has been here opened is far too large 

 to be satisfactorily examined by one worker. It is notably at 

 the commencement that the collaboration of many workers is 

 most beneficial, so that fundamental errors may be quickly 

 corrected. On this ground I respectfully invite my fellow- 

 labourers to this branch of inquiry. 



I have received through a considerable part of this inquiry 

 much valuable assistance from my friend Mr. F. H. Marshall. 



