31 



of a variety of substances, such as wax, spermaceti, and mercury, 

 with other metals, and metallic alloys, antecedent by sixteen years to 

 the first published, which I apprehend were those of Wilcke in the 

 Stockholm Transactions. The same MSS. contain also determinations 

 of the tension of vapour at low temperatures in the barometrical 

 vacuum, and an experimental demonstration, and theory, of that excess 

 of temperature in freely boiling water above the heat of steam, on 

 which observations have since been made by M. Gay Lussac and others. 



The portion of the MSS. which belongs more directly to chemistry 

 is small, with the exception of the experiments on air, but not less re- 

 markable as regards unpublished labours and hidden treasure. In 

 this point of view is to be considered a series of experiments on arsenic, 

 which bears the date of December 1764, and had been preceded only 

 by the experiments of Macquer on its neutral salt. At that early period 

 Cavendish had discovered the acid of arsenic *, had ascertained the re- 

 lation in which it stands to the oxide and the regidus, and had examined 

 the salts ivhich it forms, with at least as much accuracy as Scheele, in 

 the well-known experiments which he published in 1775 ; this treatise 

 has been twice transcribed by Cavendish from the original notes, in 

 the form of a communication to a friend, and is in a state fit for the 

 press ; nor can I conceive any reason for the suppression of experiments 

 of so much value, but that which the character ascribed to him by his 

 friends suggests. 



Among these arsenical experiments appears the first statement of the 

 nature of nitrous gas, as he afterwards described it in his paper on fac- 

 titious airs in 1766, and which was the legitimate statement, till the 

 phlogistic hypothesis was discarded: he had observed also the distinction 

 between nitrous gas and nitrous acid vapour, but was unable to assign 

 the reason of the difference. It further appears from a manuscript 

 among his early papers on factitious airs — on which he has written 

 " Communicated to Dr. Priestley " — that Cavendish was the first who 

 distinguished nitrogen from other kinds of unrespirahle and incom- 

 bustible gases, and proved by experiment that atmospheric air consists 

 of two parts, one of which in the combustion of charcoal is converted 

 into fixed air, whilst the other is a mephitic gas sui generis : Priestley, in 

 a paper in the Philosophical Transactions for 1772, mentions this com- 

 munication, but has not stated correctly the conclusions which it con- 

 tains. 



As remarkable an instance as any which I have observed of Caven- 

 dish's habit of keeping discoveries in abeyance, especially such as he 

 * Appendix, p. 52. 



