Correspondence — Mr, David Forbes, 95 



centre, it still could not have reached the exterior before, on the other hand, 

 the surface itself had also solidified and formed a crust commencing from the 

 exterior, due to the external cooling action. 



In opposition to this. Dr. Hunt states that silicates, when cold, are from one- 

 seventh to one-sixteenth part more dense than when molten, and would at once sink 

 down into the fluid mass below, and further adds that no crust could be formed 

 unless the laws of gravity were suspended. I do not know what Dr. Hunt's idea 

 of the laws of gravity may be, but I would merely again ask how far he imagines 

 a crust of sp. gr. 2 "6 could sink down into a molten sphere of a mean sp. gr. 5 •3. 

 I will not, however, repeat the other arguments which I have used in the 

 Geological Magazine, but content myself by bringing forward one not before 

 employed by me in support of my opinion. 



Some experiments which I am now engaged in, on the effect of heat on bodies 

 which contract in cooling, i.e. which are more dense when cold than when molten, 

 show in the cases tried that a body upon the first application of heat expands and 

 continues to do so up to near its melting point, when it contracts at the instant of 

 fusion ; in other words, although the substance when cold was heavier than 

 when molten, yet the same substance expanded by heat was lighter than when 

 molten. Thus, some metals were found to float about (like ice upon water) upon 

 the surface of a molten bath of the same metal into which they were placed in a 

 heated condition,^ It appears probable that the same phenomena would account 

 for such a crust as Dr. Hunt disputes, not sinking but floating on the molten bath 

 below. 



That the earth may possibly have solidified at the centre first, is not disputed by 

 me, nor does its so doing in any way affect my theoretical views. The object of 

 my observations on this head was to show that we are altogether too ignorant of 

 the character of the central mass of the earth, and of the effects likely to be 

 produced by such enormous pressures, to be enabled to reason upon such insuffi- 

 cient data with any confidence in the result. 



III. -That the original atmosphere contained "the whole of the chlorine in the 

 form of hydrochloric acid, the sulphur as sulphurous acid." 



The perusal of Dr. Hunt's remarks does not in any way tend to modify the 

 conclusions I had previously arrived at on this head. I still believe that chemists 

 will not be disposed to regard an atmosphere containing enormous volumes of 

 sulphurous acid, steam, and oxygen in excess, or in other words, which resembles a 

 great sulphuric acid chamber, as probable; and as Dr. Hunt does admit that they 

 would slowly unite to form sulphuric acid, it merely becomes a question of time 

 as to whether they united slowly or quickly. 



The arguments which I advance against supposing that such an atmosphere 

 ever did exist are, that I consider that the sulphur would unite mainly with the 

 heavier metals, and the chlorine mainly with the alkaline metals, and I conse- 

 quently infer that these elements never went into the atmosphere in any such 

 quantity as Dr. Hunt imagines. 



Dr. Hunt, in opposition, states that sulphides could not be formed, since 

 oxygen was in excess. Metallurgists know that sulphides are far less easily 

 oxidizable than is generally imagined, and that they are produced in both blast and 

 air-furnaces when the waste gases still contain unconsumed oxygen, and that time 

 is an important element in this consideration. 



But we have no proof whatever of any great excess of oxygen in the primeval 

 atmosphere ; on the contrary, we know^ that a vast amount of the oxygen now 

 present in the air must have been derived from the decomposition of the carbonic 

 acid, when the immense supplies of carbon afterwards buried in the various sedi- 

 mentary formations were extracted from the atmosphere by the action of vegetable 

 life. The slight excess of oxygen which, no doubt, was present would, further, 

 be so diffused through the enormous volume of carbonic acid, nitrogen, and 

 aqueous vapour, that it cannot be imagined to have exercised other than a most 

 feeble oxidising action. 



» As a metallurgist I have frequently observed such cases, but for a long time did not under- 

 stand the explanation ; I have to thank my friend Mr. Hackney for directing my attention to 

 the behaviour of Bessemer steel under these circumstances, as it gives great trouble to the work- 

 men by persistently floating high on the surface of the melted steel (even when in pieces of 

 40 lbs. or more) as long as its temperature is below its fusing point. 



