22 REPORT— 1902. 



About this time a remarkable observation was made by Professor 

 Braun at Moscow, who, during the severe winter of 1759, succeeded in 

 freezing mercury by the use of a mixture of snow and nitric acid. When 

 we remember that mercury was regarded as quite a peculiar substance 

 possessed of the essential quality of fluidity, we can easily understand the 

 universal interest created by the experiment of Braun. This was accen- 

 tuated by the observations he made on the temperature given by the 

 mercury thermometer, which appeared to record a temperature as low as 

 minus 200° C. The experiments were soon repeated by Hutchins 

 at Hudson's Bay, who conducted his work with the aid of suggestions 

 given him by Cavendish and Black. The result of the new observations 

 Avas to show that the freezing-point of mercury is only minus 40° C, the 

 errors in former experiments having been due to the great contraction of 

 the mercury in the thermometer in passing into the solid state. From 

 this it followed that the enormous natural and artificial colds which had 

 generally been believed in had no proved existence. Still the possible 

 existence of a zero of temperature very different from that deduced from 

 gas thermometry had the support of such distinguished names as those of 

 Laplace and Lavoisier. In their great memoir on ' Heat,' after making 

 what they consider reasonable hypotheses as to the relation between 

 specific heat and total heat, they calculate values for the zero which range 

 from 1,500 to 3,000 degrees below melting ice. On the whole, they regard 

 the absolute zero as being in any case 600 degrees below the freezing- 

 point. Lavoisier, in his ' Elements of Chemistry ' published in 1792, goes 

 further in the direction of indefinitely lowering the zero of temperature 

 when he says, ' We are still very far from being able to produce the degree 

 of absolute cold, or total deprivation of heat, being unacquainted with any 

 degree of coldness which v;e cannot suppose capable of still further augmen- 

 tation ; hence it follows we are incapable of causing the ultimate particles 

 of bodies to approach each other as near as possible, and thus these particles 

 do not touch each other in any state hitherto known.' Even as late as 

 the beginning of the nineteenth century we find Dalton, in his new system 

 of ' Chemical Philosophy,' giving ten calculations of this value, and adopt- 

 ing finally as the natural zero of temperature minus 3,000° C. 



In Black's lectures we find that he takes a very cautious view with 

 regard to the zero of temperature, but as usual is admirably clear with 

 regard to its exposition. Thus he says, ' We are ignorant of the lowest 

 possible degree or beginning of heat. Some ingenious attempts have been 

 made to estimate what it may be, but they have not proved satisfactory. 

 Our knowledge of the degrees of heat may be compared to what we should 

 have of a chain, the two ends of which were hidden from us and the 

 middle only exposed to our view. We might put distinct marks on some 

 of the links, and number the rest according as they are nearest to or 

 further removed from the principal links ; but not knowing the distance 

 of any links from the end of the chain we could not compare them together 

 with respect to their distance, or say that one link was twice as far from 



