436 Forbes — ChemiBtry of the Primeval Earth. 



Dr. Hunt then tells us, that the cooling of the liquid globe would 

 be " like the cooling of a great bath of metal or sulphur," and that 

 it " would commence at the centre and extend outwards toward the 

 surface." This may fairly be questioned. Any one who ever cast a 

 bullet knows how long the centre remains fluid after the exterior 

 shell is solid ; the founder knows how difficult it is to get his cast- 

 ings perfect, with their centres solid; and the mode of causing metals, 

 as bismuth, lead, etc., etc., to crystallize, by allowing as large a 

 mass of the fluid metal as possible to solidify exteriorly, and then 

 pricking a hole through the crust, so as to let out the still fluid cen- 

 tral metal, is familiar to all, as well as, probably, the fact that this 

 same proceeding is commonly employed to obtain crystals of 

 sulphur. 



But Dr. Hunt will doubtless object, that the above experiments 

 were not made under pressure, and, to demonstrate that solidifi- 

 cation must in such case have commenced from the centre, refers 

 to the experiments of Hopkins and Fairbairn, as proving that the 

 fusing points of bodies are elevated by pressure. 



Fully aware that the fusing points of certain bodies are actually 

 raised when under pressure, and prepared also to believe that 

 silicates may be amongst such, still the author, after a careful 

 analysis of both Hopkins' experiments,^ and those of Bunsen,- can- 

 not find such conclusive evidence in the same, as Dr. Hunt in his 

 lecture would lead his audience to infer. 



The results of these experiments condensed into a few words are 

 as follows : — Hopkins and Bunsen both find by experiment that 

 certain organic compounds (spermaceti, wax, stearine, and paraffine) 

 actually have their melting and freezing points elevated (but in a very 

 irregular manner) , when exposed to pressures of from 1 to 793 atmos- 

 pheres (15 to 11,880 lbs. per square inch)'; further, Hopkins shows 

 that sulphur has likewise its fusing point elevated by a pressure 

 from 1 to 520 atmospheres (15 to 7,790 lbs. per square inch), but that 

 after this up to 793 atmospheres (11, 880 lbs. per square inch), the 

 highest pressure tried, the ratio of increase of temperature to pres- 

 sure diminishes greatly, which might be supposed to indicate that at 

 still higher pressures the fusing jjoint might again diminish and 

 even become dejDressed instead of elevated. No allusion is made to 

 Mr. Hopkins' statement that in the case of such metallic alloys as he 

 had tried, " that lie had not detected any elevation of fusing temperature 

 acquired by increasing the pressure." 



Even allowing however that the fusing points of bodies in general 

 are elevated by pressure, it does not necessarily follow that the 

 centre of the earth must have solidified before or even at the same 

 time with the crust, unless it is also taken for granted that the 

 earth's mass is perfectly homogeneous throughout, or at least com- 



^ Brit. Association Report, 1854, p. 57. 



2 Poggendorf. Ann., vol. 81, p. 5(i2. 1850. 



^ The nature of such organic substances being totally different from any to be met 

 ■with under the circumstances here under consideration, much dependence could not 

 be placed upon the similarity of behaviour of inorganic compounds. 



