298 THE NORTHERN MICROSCOPIST. 
they are the only cell to be depended upon for size and shape. I 
never use any other. My plan of using these caps is as follows :— 
After having fixed the cover properly and without leakage, I wash 
the preparation under the tap until all traces of glycerine are 
removed, then run a good thick ring of any kind of cement round 
the edge of the cover and cell, finally dropping on the cap, when 
the mount should be placed aside for a week, so that the cement 
or varnish may properly set. I use these caps for all deep cells, as 
they prevent the cover from being pushed off, and am having some 
made half the depth of those sent, for shallow cells.—Fred. Enock. 
[We are glad to find Mr. Enock doing something to improve the 
condition of glycerine mounts. The caps he has sent us will 
certainly do much to prevent leakage and disappointment. ‘They 
are made to fit Pumphrey’s vulcanite cells of 2”, 9’, 3’, and 2” 
diameters, and are sold at the moderate price of threepence per 
dozen.—Ep. | 
FRESH-WATER SPoNGE.—A short time ago I had the pleasure of 
communicating to S. G. the occurrence of Daphnia Bairdi and 
Diaptomous castor in this locality. Since then I have found a 
Sponge, which seems to differ slightly from the ordinary Spongzlla 
fiuviatilis. One of the specimens was over three inches square, 
which so far as I know is a very unusual size. Hogg states that 
the gemmules of Spongilla have a skeleton of bi-rotulate spicula. 
I have not been able to detect any spicula coating the gemmules 
even when treated with acetic acid. These two points, large size 
and absence of spicula in the gemmules, lead me to think it may 
possibly be new.—/. £. Lord. 
Cuas. A. SPENCER, the celebrated American objective manu- 
facturer, is dead. He died at his residence in Geneva, N. Y., on 
the 28th of September last. He was born at Canastota, N. Y., in 
the year 1813, and was educated at the Cazenovia Academy, from 
which he graduated. He afterwards spent one year at Hobart 
College. During the past few years he did little more than 
superintend the labor of his sons, and perhaps the latest produc- 
tions of his factory were mainly due to the abilities of his son 
Herbert, who now carries on the manufacture, under the title of 
Herbert R. Spencer and Co. 
AguariA.—Could any of your readers tell me how often it is 
necessary to clean out an aquarium, and how many fish, snails, and 
weeds are necessary ?—Zom Brown. 
KEEPING Rautin’s FLurp.—Both Pasteur’s and Raulin’s fluids 
are very difficult to keep. They are so extremely sensitive that 
the simple exposure of the fluids to air for two or three days (in a 
town) is sufficient to convert the whole into a ropy mass of 
