VOL. LXXIII.] PHILOSOPHICAL TBANSACTIONS. 453 



connected view of the different observations and experiments in any one branch 

 of science, we are furnished with the best opportunity of discriminating what is 

 certain from what is doubtful, and acquire as distinct ideas as the actual state of 

 knowledge will admit. On the present subject of mercurial congelation, the 

 conclusions have in general been noticed, as the premises occurred. Though 

 Mr. Hutchins's experiments did not stand in need of any confirmation, yet still 

 it is pleasant to see their principal result, the freezing point of quicksilver, esta- 

 blished by such a body of collateral evidence as, taken together, is absolutely 

 irresistible. But besides the information obtained relative to quicksilver itself, 

 we have been able to correct several vulgar prejudices. The difference between 

 cold climates no longer appears so prodigious, nor the resisting powers of animals 

 and vegetables so astonishing and inconceivable. That extensive scale of heat, 

 which represents its diminutions by artificial means as continued down so many 

 hundreds of degrees below the greatest produced by nature, however specious in 

 prospect, proves to be destitute of foundation. The use of quicksilver for ther- 

 mometers is at length fully ascertained. From the boiling point, to 39 or 40° 

 below O, it must be considered as unexceptionable, all suspicion of its irregular 

 contraction within those bounds being removed, by such a complete explanation 

 of the cause on which its anomalous descent in the lower part of the scale de- 

 pends. On this principle there might perhaps, adds Dr. B., be some propriety 

 in constructing thermometers of mercury, to fix the cypher at the point of con- 

 gelation, and thence reckon the degrees of heat upwards. 



XXII. Experiment relating to Phlogiston, and the seeming Conversion of Water 



into Air. By Joseph Priestley, L L. D., F. R. S. p. 3Q8. 

 This paper may be more advantageously consulted in the collection of Dr. 

 Priestley's various writings on air and other branches of natural philosophy. It 

 is the 1st article in the 6th or last volume of that collection, printed in 8vo. at 

 Birmingham, 1786, where it is enriched with additional notes. 



XXIII. Description of an Improved Air- Pump. By Mr. T. Cavallo., F.R.S. 



p. 435. 



The principal improvements which the air-pump received since it was first 

 invented, were contrived by Mr. Smeaton, p. r. s., and are described in the 47 th 

 volume of the Philos. Trans. This gentleman considering the imperfections of 

 the air-pumps usually made, not only found means to correct several of them, 

 but improved almost every part of the machine, so as to render it far superior to 

 any thing of the kind done before. It appears, by some experiments of Mr. 

 Nairne, f. r. s. described in the 47th volume of the Phil. Trans, which were 

 made with an air-pump constructed after Mr. Smeaton's principle, that by means 



