Diatomacee. 51 
to a kind of cushion, as it very frequently is—not to the highest 
point by the cushion, but to the lowest, the part in contact 
with the slide, which is for the time the bed of its pond or ditch, 
leaving its upper part a perfectly transparent skeleton with all 
its markings visible, and having at its lower part all its 
granules closely packed in the oily protoplasmic contents. 
If the species be naviculoid, the contents will go half to one 
end, and half to the other, and be packed in a similar manner. 
Tribes of men and animals become extinct, but so long as the 
present circulation of water between earth and air continues, 
so long the diatom will continue to exist, to store up, and to 
secrete the matter which the intelligent traveller who has 
climbed nature’s fairest peaks knows once formed an integral 
portion of the mighty masses on which he has gazed with 
admiration and awe, but which has by this circulation found 
its way into all our rivers, ponds, and ditches. 
I feel I have already taxed too much your powers of endurance, 
but will, if you wish it, say a few words on the mounting and pre- 
servation of the diatoms when collected. This branch of the 
subject assumes different proportions, according to the object 
in view. If the skeleton only be wanted, the collection should 
be placed in a small glass of water and stirred. Weighty 
particles will soon subside, and the water should be poured 
into a large glass. This process may be repeated till the 
larger glass be full. Allow it to stand three or four hours or 
more. Pour off the liquid and you will have what you want 
at the bottom. Refill the vessel with water and allow a 
second settlement of the same duration. One or more 
washings might also be made, but when you have poured off 
nearly all the water from the last, return the gathering to the 
smaller glass, and allow it to subside in the same way. Pour 
off the water and add slowly a small quantity of hydrochloric 
acid. There will, in all probability, be effervescence, and the 
contained matter will rise to the surface. When the action 
has nearly ceased, add a little more acid. Let it remain ina 
safe place till the whole has subsided. You may then—it will 
take some time—place the glass over the flame of a spirit 
lamp till you see that all action has ceased. Allow time to 
subside, pour off the acid, and repeat the process with nitric 
acid, testing occasionally by taking, with a small tube, a small 
quantity for observation under the microscope. This is the 
most troublesome part of the whole business, and requires 
great care. It should be performed in some outhouse, and the 
- operator should remember that these acids, employed to destroy 
vegetable and other matter, will also destroy cloth. If the 
outhouse be supplied with gas, so much the better. The 
glass containing all can be placed on wire gauze, at a safe 
