186 MANAGEMENT OF THE MICKOSCOPE. 



division, exhibit a like movement, so that it cannot be regarded 

 as indicative of any endowment pecuUar to the fovilla-granules ; 

 and subsequent researches have shown, that there is no known ex- 

 ception to the rule, that such motion takes place in the particles of 

 all substances, though some require to be more finely divided 

 than others, before they will exhibit it. Nothing is better adapted 

 to show it, than a minute portion of gamboge, indigo, or carmine, 

 rubbed up with water ; for the particles of these substances, which 

 are not dissolved, but only suspended, are of sufficiently large 

 size to be easily distinguished with a magnifying power of 250 

 diameters, and are seen to be in perpetual locomotion. Their 

 movement is chiefly of an oscillatory kind ; but they also rotate 

 backwards and forwards upon their axes, and they gradually 

 change their places in the field of view. It may be observed that 

 the movement of the smallest particles is the most energetic, and 

 that the largest are quite motionless, whilst those of intermediate 

 size move, but with comparative inertness. The movement is 

 not due (as some have imagined) to evaporation of the liquid ; 

 for it continues, without the least abatement of energy, in a drop 

 of aqueous fluid that is completely surrounded by oil, and is 

 therefore cut off" from all possibility of evaporization ; and it has 

 been known to continue for many years, in a small quantity of 

 fluid enclosed between two glasses in an air-tight case. It is, 

 however, greatly accelerated, and rendered more energetic, by 

 Heat ; and this seems to show that it is due, either directly to 

 some calorical changes continually taking place in the fluid, or 

 to some obscure chemical action between the solid particles and 

 the fluid, which is indirectly promoted by heat. It is curious 

 that the closer the conformity between the specific gravity of the 

 solid particles and that of the liquid, the less minute need be 

 that reduction in their size which is a necessary condition of their 

 movement ; and it is from this that the substances just named 

 are so favorable for the exhibition of it. On the other hand, the 

 particles of metals, which are from seven to twelve times as heavy 

 as water, require to be reduced to a minuteness many times 

 greater than that of the particles of carmine or gamboge, before 

 they become subject to this curious action. In any case in which 

 the motions of very minute particles, of whatever kind, are in 

 question, it is necessary to make allowance for this " molecular 

 movement;" and the young Microscopist will therefore do well 

 to familiarize himself with its ordinary characters, by the careful 

 observation of it in such cases as those just named, and in any 

 others in which he may meet with it. 



101. Comparative Values of Object- Glasses; Test Objects. — In 

 estimating the comparative values of different object-glasses, re- 

 gard must always be had to the purpose for which each is de- 

 signed ; since it is impossible to construct a combination, which 

 shall be equally serviceable for every requirement. It is com- 

 monly assumed, that an Objective which will show certain test- 



