120 Rev. W.V. Harcourt on Lord Brougham's statements 



the air was not amiss*;" and the experiments of the great 

 Italian philosopher, which laid the original foundation of all 

 our knowledge of elastic fluids, ought not to have been en- 

 tirely forgotten by any one who appreciates duly those capital 

 discoveries by which the ideas of men are fixed and a new 

 order of facts is ascertained. 



To Black, on the other hand, with like even-handed justice, 

 you ascribe a knowledge of the lightness of hydrogen and the 

 heaviness of carbonic gas, which you have no ground for sus- 

 pecting him to have possessed. Experiments, indeed, had 

 been made with a view of ascertaining such points, and your 

 assertion, that "Cavendish first set the example of weighing 

 permanently elastic gases," is so far from the truth, that the 

 factitious gases themselves had been weighed both by Hawks- 

 bee and Hales. Hair- weighed the " air of tartar," which 

 consists of a mixture of carburetted hydrogen and carbonic 

 gases in a bladder, and then filling it with common air com- 

 pared the weights t; Hawksbee ascertained accurately the 

 specific gravity of air that had passed through tubes filled with 

 iron wires, and heated red in the fire, which consisted partly 

 of carbonic acid and partly of nitrogen %. But these mixed 



* " Sa facon tie peser l'air n'est pas mauvaise, si tant est que la pesanteut* 

 en soit si notable qu'on la puisse apercevoir par ce moyen; mais j'en 

 doute." (CEuvres de Descartes, torn. vii. p. 440.) Thus Descartes wrote 

 to Mersenne in 1638. In 1642 he repeated the experiment himself by a 

 method far less susceptible of accuracy, and obtained a result much further 

 from the truth, which satisfied him however, " que la poids de l'air est 

 sensible en cette facon." {QZuvres, torn. viii. p. 567.) Dr. Whewell has 

 taken notice (History of Mechanics, p. 66) that " in a letter of the date of 

 1631 he (Descartes) explains the suspension of mercury in a tube closed at 

 the top by the pressure of the column of air reaching to the clouds." In this 

 letter the atmosphere is compared to a pack of wool, the filaments of which 

 are all heavy, and press on each other from the clouds to the earth, being 

 only kept apart by the aether which plays between them, " ce qui fait un 

 grand pesanteur" — expressions which at first sight might lead to the idea 

 that he had anticipated the theory of the elevation of the barometric 

 column j but it is evident from many subsequent letters of Descartes, that 

 he had no correct conception of the statical pressure of fluids, and was 

 therefore incapable of reasoning justly on this subject. The tube in which 

 the mercury was suspended in the casein question, was a straight tube with- 

 out a bason: he tried to account for the phenomenon of its suspension on 

 his principle of circular movement in a plenum, by supposing that the mer- 

 cury, before it could quit the tube, must effect the circle of motions re- 

 quired to bring down from the sky a current of aether to supply the vacuum 

 left at the top of the tube by the descent of the quicksilver ; and pre- 

 suming the column of air which it had to lift to be as heavy as itself, he 

 concluded that no such circular motion in the chain of matter could take 

 place. It is possible however that this representation of the atmosphere 

 as a heavy column may have conduced to suggest the more correct views 

 of the subject afterwards adopted. 



f Veg. Status, p. 185. { Phil. Trans., No. 328, p. 199. 



