of this liquid be the same in both tubes. The globe and its stop-cock are now removed, and in their place is put the funnel c, furnished with a stop-cock a, which differs in its construction from the ordinary stop-cocks. It is not pierced through and through, but has only a small cavity as seen at o on the right of the figure. Having poured into the funnel o the liquid which is to be vaporised, having marked the level I of the mercury and opened the stop-cock b, the stop-cock a is then turned in such a manner that the cavity is filled with the liquid; it is then turned again, so that the liquid may enter the space A, and there be vaporised. Thus the liquid is made to enter, drop by drop, until the air in the tube is saturated with vapour, which is ascertained by the level ceasing to descend. As the tension of the vapour produced in the space A is added to that of the air already there, the volume is increased; but it is easily reduced to its original volume, by pouring an additional quantity of mercury into the tube B. When the mercury is by this means made to rise in the large tube to the level I which it had at first, there is observed in the tubes B and A a difference of level Bo, which evidently represents the tension of the vapour which has been produced; for the air having resumed its original volume, its tension is not changed. Now if some drops of the same liquid which was introduced into the space A be passed into the barometric vacuum, a depression exactly equal to Bo is observed; and this proves clearly that, at the same temperature, the tension of a vapour is the same in a gas as in a vacuum. As to the second law, it is proved by the preceding experiment, because when the mercury has resumed its level 1, the mixture supports the atmospheric pressure which acts at the top of the tube F, plus the weight of the column of mercury BO. Now these two pressures exactly represent, the one the tension of the dry air, and the other the tension of the vapour. Whence, the second law may be considered as a consequence of the first. The apparatus which we have described only admits of experiments at the ordinary temperature; but M. Regnault, by means of an apparatus capable of being employed at different temperatures, has compared the tension of the vapour of water in air and in a vacuum. He found that it was always feebler in the former than in the latter case; but the differences were so trifling, that the law of Gay-Lussac is not diminished in its generality and value, Fig. 191. Applications.-In addition to the well-known application of steam, it has often been proposed to employ vapours and gases of various kinds, as a moving power; as, for instance, by the expansion of heated air, or by the alternate vaporisation and liquefaction of different substances, such as ether, carbonic acid, etc. It may be useful, however, to mention here an important principle announced for the first time in 1824, by M. S. Carnot, in a small but curious work, entitled Reflexions sur la puissance mécanique du feu, viz. that the same It is composed of a glass tube A, to the extremities of which quantity of heat can only are cemented two iron stop-cocks b and d. The lower stop- produce the same amount of labour, whatever may be the cock is furnished with a short tube, which puts the tube A in nature of the gas on which it acts, provided that no loss is communication with a second tube B of smaller diameter. A occasioned by improper or defective arrangements. Experience scale is placed between these two tubes, in order to measure has fully confirmed the theoretical considerations on which the height of the column of mercury contained in each. The this principle was founded by M. Carnot. In fig. 191, is repretube A is then filled with dry mercury, and the stop-cocks bsented a sort of jack or turnspit, which is very common in and d being shut, we first screw on the stop-cock 6, at the several parts of the continent, and which is put in motion by place of the funnel c, a glass globe filled with dry air or any other the current of the air which ascends the chimney in consegas, and furnished also with a stop-cock which is closed. quence of the continual rarefaction of the lower strata which Next, opening the three stop-cocks, a part of the mercury is it contains, by the heat of the fire. Its mechanism is so allowed to flow from the tube A, which is replaced by dry air evident as to require no particular description. This apparafrom the globe. The stop-cocks are then closed, and as the tus is described in the MSS. of Leonardo Da Vinci, who died air in the space a expands on issuing from the globe, and is in 1519, and its invention and use probably originated in a under a pressure less than that of the atmosphere, it is forced much remoter period. back by pouring some mercury into the tube B, until the level Reflections on the Mechanical Power of Fire. POESY. Divinest Poesy!-'tis thine to make Age young,-youth old,-to baffle tyrant Time; From antique strains the hoary dust to shake, And with familiar grace to crown new rhyme. Long have I loved thee,-long have loved in vain, The lovely images of earth and sky From thee I learned within my soul to treasure; Not Fortune's spite, nor hopes that once have been- Of pregnant ills, and penitential harms, léon. i -C'est nous, dit Napoléon, qui avons cassé ses œufs en courant après les papillons; 10 et je mène cette petite à ma man pour qu'elle paici le dégât que nous avons fait." Peu d'instants après, la bonne et les deux enfants, suivis de la petite paysanne entrèrent dans une salle où était réunie la famille Bonaparte. Madame Lotitia prit la parole: 12 -Napoléon, Elisa, je vous avais fait cadeau d'un filet;13 mais vous m'avez désobéi en franchissant la haie et en courant plus loin à travers la campagne; rendez-moi vos filets, cela vous épargnera l'occasion de me désobéir encore. 14 -Maman, fit Napoléon, c'est moi qui suis coupable; c'est moi qui ai entraîné Elisa.15 La petite fille ne dit mot, mais elle sauta au cou de son frère.16 Ma sœur, dit l'archidiacre d'Ajaccio, péché avoué est à moitié pardonné: je demande grâce pour Napoléon. 17 Oh! bien mon oncle, dit Elisa, demandez grace aussi pour moi, je vous en prie, car j'ai fait bien plus de mal que lui. 18 -Et quel si gros péché as-tu donc commis ?19 dit le vicillard vénérable en souriant;' parle franchement, et je te promets d'intercéder pour toi. 20 -Maman, dit alors Napoléon, j'ai encore un grâce à vous demander. Vous me donnez dix sous par semaine œufs de cette pauvre petite qui attend là ce que tout cela pour mes menus plaisirs. Eh bien! achevez de payer les va devenir, et vous ne me donnerez plus rien jusqu'à ce que nous soyons quittes. -D'accord, dit Madame Lotitia en faisant approcher la petite paysanne, et lui donnant un petit écu. Napoléon,o en voilà pour six semaines. les deux pièces de monnaie qu'elle avait reçues de lui au -L'enfant courut à Napoléon, et voulut lui remettre moment où l'accident était arrivé; mais il refusa. terrogea la petite paysanne." Cette probité pluti à Madame Bonaparte,' qui alors in10 Elle apprit que c'était la fille d'un pauvre pêcheur, que sa mère était malade," qu'elle demeurait dans une chétive cabane, 12 sur le bord de la mer, à quelque distance 13 de l'endroit où son panier avait été renversé. -Ta mère est malade, dis-tu, mon enfant ? 14 elle n'a pas de médecin qui la soigne, sans doute. J'irai la voir. tout de suite.15 Nous reconduirons Charlotte. -Oh! maman, je vous en prie, s'écria Napoléon, allons-y -Volontiers, répondit Madame Bonaparte. Allons,16 mes enfants, partons. Les enfants ne se le firent pas répéter. Quelques instants après, ils arrivèrent au pied d'un rocher. 17 |