Molecular Actions of Crystalline Particles, fyc. 99 



second, water as in mists, fogs, &c. ; and of the third, the va- 

 pours of arsenic and of corrosive sublimate. Bodies in either 

 of these conditions possess the faculty of assuming a definite 

 crystalline form on becoming solid. The properties of the 

 gaseous vapours are so well known, that it is unnecessary to 

 dwell upon them here. 



The second class, or the liquid globular vapours or fumes, 

 which, as we have said, cause those accumulations known 

 under the name of fogs, clouds, or mists, are those which I 

 intend at present to examine, as they comprehend the theory 

 of the fixation of the mercurial vapours in the Daguerreotype. 

 It was formerly believed that vapour or mist was composed of 

 minute spherules or globules of liquid water, and in Newton's 

 works we find evidence that such was his opinion. According 

 to another view, first advanced I believe by De Saussure, these 

 vapours were composed of vesicles or very minute bubbles, 

 exactly resembling, on a small scale, the common soap-bubble. 

 This opinion has received the assent of Fresnel and Berzelius, 

 and at present obtains general credence. The proofs on which 

 it is considered to be founded, are principally the observa- 

 tions of De Saussure, who asserts that on high mountains, or 

 in the clouds, he has been able to detect these air-vesicles 

 with the naked eye, and has seen them burst as they came in 

 contact with each other. Berzelius recommends the exami- 

 nation of the vapour of water over a dark surface, such as that 

 of ink, with a lens of a short focus. He says, that vesicles 

 may be detected in this manner, varying in size from -^j-^-q to 

 2tVo^ 1 °f an ' nc h> which occasionally burst as they touch 

 each other. The suspension of clouds is also used as an ar- 

 gument in favour of the vesicular theory, as it is contended 

 that liquid spherules would descend to the ground by their 

 specific gravity in such situations. Fresnel indeed compares 

 the globules to small balloons, which dilate or contract, accord- 

 ing to the temperature of the air they contain. 



A few days' stay at the convent of St. Bernard gave me an 

 opportunity of repeating the observations on the clouds, as 

 mentioned by De Saussure, which may be also made in this 

 season on our London fogs. Globules of various sizes in these 

 circumstances are frequently discerned by the naked eye float- 

 ing in all directions. I have endeavoured to ascertain their 

 vesicular structure, but have been unable to do so from direct 

 observations. It is frequently a most difficult point, in mi- 

 croscopic investigation, to decide upon the existence of a thin 

 transparent membrane. It is still more so to pronounce upon 

 the vesicular or spherular structure of globules in constant 

 agitation ; and I believe that if minute spherules and vesicles 



