3 FAMILIAR LETTERS ON CHEMISTRY. 



wax before the flame of the oxy-hydrogen blowpipe. On the other hand, of the 

 twenty-eight gaseous bodies with which we are acquainted, twenty-five may be 

 reduced to a liquid state, and one into a solid. Probably, ere long, similar changes 

 of condition will be extended to every form of matter. 



There are many things relating to this condensation of the gases worthy of 

 your attention. Most aeriform bodies, when subjected to compression, are made 

 to occupy a space which diminishes in the exact ratio of the increase of the com- 

 pressing force. Very generally, under a force double or triple of the ordinary 

 atmospheric pressure they become one half or one third their former volume. This 

 was a long time considered to be a law, and known as the law of Marriotte ; but a 

 more accurate study of the subject has demonstrated that this law is by no means 

 of gen eral application. The volume of certain gases does not decrease in the ratio 

 of the increase of the force used to compress them, but in some, a diminution of 

 their bulk takes place in a far greater degree as the pressure increases. 



Again, if ammoniacal gas is reduced by a compressing force to one sixth of 

 its volume, or carbonic acid is reduced to one thirty-sixth, a portion of them loses 

 entirely the form of a gas, and becomes a liquid, which, when the pressure is 

 withdrawn, assumes again in an instant its gaseous state another deviation from 

 the law of Marriotte. 



Our process for reducing gases into fluids is of admirable simplicity. A simple 

 bent tube, or a reduction of temperature by artificial means, have superseded the 

 powerful compressing machines of the isarly experimenters. 



The cyanuret of mercury, when heated in an open glass tube, is resolved into 

 cyanogen gas and metallic mercury ; if this substance is heated in a tube hermeti- 

 cally sealed, the decomposition occurs as before, but the gas, unable to escape, and 

 shut up in a space several hundred times smaller than it would occupy as gas 

 under the ordinary atmospheric pressure, becomes a fluid in that part of the tube 

 which is kept cool. 



When sulphuric acid is poured upon limestone in an open vessel, carbonic acid 

 escapes with effervescence as a gas ; but if the decomposition is effected in a strong, 

 close, and suitable vessel of iron, we obtain the carbonic acid in the state of liquid. 

 In this manner it may be obtained in considerable quantities, even many pounds 

 weight. Carbonic acid is separated from other bodies with which it is combined as 

 a fluid under a pressure of thirty-six atmospheres. 



The curious properties of fluid carbonic acid are now generally known. When 

 a small quantity is permitted to escape into the atmosphere, it assumes its gaseous 

 state with extraordinary rapidity, and deprives the remaining fluid of caloric so 

 rapidly that it congeals into a white crystalline mass like snow : at first, it was 

 indeed thought to be really snow, but upon examination it proved to be pure frozen 

 carbonic acid. This solid, contrary to expectation, exercises only a feeble pressure 

 upon the surrounding medium. The fluid acid enclosed in a glass tube rushes at 

 once, when opened, into a gaseous state, with an explosion which shatters the tube 

 into fragments ; but solid carbonic acid can be handled without producing any 

 other effect than a feeling of intense cold. The particles of the carbonic acid being 

 so closely approximated in the solid, the whole force of cohesive attraction (which 

 in the fluid is weak) becomes exerted, and opposes its tendency to assume its 

 gaseous state ; but as it receives heat from surrounding bodies, it passes into gas 

 gradually and without violence. The transition of solid carbonic acid into gas 

 deprives all around it of caloric so rapidly and to so great an extent, that a degree 

 of cold is produced immeasurably great, the greatest indeed known. Ten, twenty, 

 or more pounds weight of mercury, brought into contact with a mixture of ether 

 and solid carbonic acid, become in a few moments firm and malleable. This, how- 

 ever, cannot be accomplished without considerable danger. A melancholy accident 

 occurred at Paris, which will probably prevent for the future the formation of 

 solid carbonic acid in these large quantities, and deprive the next generation of 

 the gratification of witnessing these curious experiments. Just before the com- 

 mencement of the lecture in the Laboratory of the Polytechnic School, an iron 

 cylinder, two feet and a half long and one foot in diameter, in which carbonic acid 

 had been developed for experiment before the class, burst, and its fragments were 

 scattered about with the most tremendous force ; it cut off both the legs of the 

 assistant and killed him on the spot. This vessel, formed of the strongest cast iron, 

 and shaped like a cannon, had often been employed to exhibit experiments in the 

 presence of the students. We can scarcely think, without shuddering, of the 

 dreadful calamity such an explosion would have occasioned in a hall filled with 

 spectators. 



When we had ascertained the fact of gases becoming fluid under the influence 

 of cold or pressure, a curious property possessed by charcoal, that of absorbing 



