178 HOW TO WORK 



become stony, so as to produce what may be called glass- or stone- 

 cai'itics. All these kinds of cavities can readily be seen with suitable 

 magnifying powers, and distinguished from each other by various 

 definite peculiarities. 



From these and other facts, the following conclusions were 

 deduced : 



1. Crystals containing only cavities with water were formed from 

 solution. 



2. Crystals containing only stone- or glass-cavities were formed 

 from a state of igneous fusion. 



3. Crystals containing both water- and stone- or glass-cavities 

 were formed, under great pressure, by the combined influence of 

 highly heated water and melted rock. 



4. That the relative amount of water present in the cavities may, 

 in some cases, be employed to deduce the temperature at which the 

 crystals were formed, since the accompanying vacuity is due to the 

 contraction of the fluid on cooling. 



5. Crystals containing only empty cavities were formed by sub- 

 limation, unless the cavities are fluid-cavities that have lost their 

 fluid, or are bubbles of gas given off from a substance which was 

 fused. 



6. Crystals containing few cavities were formed slowly, in com- 

 parison with those of the same material that contain many. 



7. Crystals that contain no cavities were formed very slowly, or by 

 the cooling from fusion of a pure, homogeneous substance." 



Independent of L their connection with the origin of rocks, 

 these fluid-cavities are very interesting as microscopical objects, since 

 the small bubbles which they contain exhibit spontaneous molecuhr 

 movement to great advantage. As illustrations of such cavities, the 

 reader is referred to figs. 263, 264, in pi. XLII. In fig. 263 is one 

 enclosed in a crystal of nepheline, from one of the ejected blocks of 

 Monte Somma, and shows two different kinds of included crystals, 

 water or rather a concentrated saline solution and a spherical 

 bubble. Fig. 264 is from the quartz of granite, with a very small 

 bubble, which moves about freely in the water filling the cavity. It is 

 only when such bubbles are very minute that their movement is 

 decided, but when, for example, about -5-o^o^th inch in diameter, 

 they frequently as it were swim about in the liquid, like minute ani- 

 malcules. Brown, in a paper printed in 1827, showed that minute solid 

 or even liquid particles contained in another liquid possess a natural 

 molecular movement, quite independent of any currents, and this 

 motion of the bubbles in fluid-cavities appears to me to be in all 

 respects the same, only that it is the movement of a gaseous globule. 



