114 B. MALLET ON THE TEMPEEATUKE OF THE 



the results given in a series of memoirs, of an elegance and^ 

 accuracy not to be surpassed, published in the ' Memoirs of the. 

 Academy of Sciences of rran<5e.' The experiments were per- 

 formed by order and at the expense of the French Government; 

 and the results which the memoirs contain remain to the present 

 hour the most accurate information we possess as to the relations 

 between temperature and pressure of aqueous vapour or steam. 

 Eegnault's experiments proeeeded as far as a steam-pressure of 24. 

 atmospheres ; and, by extrapolation and empirical formulae, the re-, 

 suits have been extended up to a pressure of 50 atmospheres or: 

 beyond. These formulae, and the results to which they lead, have 

 not the same authority they seemed at first to possess ; for almost 

 nothing was then known as to the change in relation between tem- 

 perature and pressure at some point of temperature differing with 

 the constitution or chemical character of the evaporating body, which, 

 although suggested long ago by the remarkable experiments of 

 Cagniard de la Tour, attracted but little attention until the publica- 

 tion of Dr. Andrews's researches in our own day, by whom this 

 change in the relation of temperature and pressure has been called 

 the " critical point." 



It has been understood that Dr. Andrews has been engaged in 

 endeavouring to ascertain experimentally at what temperature the 

 " critical point " for water is reached. The inquiry is, however, one 

 of great experimental difficulty ; and I am not aware that any result 

 has yet been arrived at. Some analogies, though not of a very pre- 

 cise character, suggest the supposition that it may be found at about 

 the temperature of melted zinc, or about 700° Pahr. Even were 

 this the extreme limit of temperature at which liquid water first 

 reached our earth's surface, it would have been exalted enough to 

 have given rise to many remarkable geological phenomena, such as 

 are touched upon further on. 



The experiments of Eegnault need therefore to be extended, and 

 the extension of his results by calculation to be revised, before we 

 shall be in a position to know with sufficient certainty what would 

 be the temperature of the last portions of water were the ocean 

 boiled dry and all the water in the state'of vapour still floating over 

 our globe, or, what is the same thing, what would be the tempera- 

 ture of the water first deposited from such an atmosphere upon the 

 heated earth. While I have thus been able, I hope, to make clear 

 the line of physical argument by which, in general terms, these im- 

 portant deductions have been arrived at, it is plain that precise 

 numerical values cannot be attached to them until we shall have as- 

 certained, approximately at least, the actual volume of water existing 

 upon our globe, and shall also possess experimental information as 

 to the relation between temperature and pressure of steam extending 

 beyond Eegnault's Hmits. 



So many circumstances concur in support of the nebular hypo- 

 thesis, that it seems to deserve rather the title of the nebular theory ; 

 but, in whichever light it be viewed, there must have been a time 

 when the surface of our planet was destitute of water, and when aU 



