BY A. MEAD EDWARDS. 25 



acid upon tlie organic matter of the diatoms ceases. There- 

 after, and while the liquid is still hot, I have found it often 

 advantageous to drop in one or two fragments of bichromate 

 of potash. The organic matter is more thoroughly destroyed in 

 this way than when the acid is used alone. Thereafter it is 

 well to pour the acid and diatoms into a capacious beaker of 

 clean water, washing the tube or smaller beaker out with a little 

 water, and adding this to the other. After the diatoms have 

 all settled, which will often require hours, the supernatant 

 fluid is carefully poured off, and a fresh supply added; and this 

 must be repeated several times until all of the acid and colored 

 chromium compound has been removed. When this point is 

 arrived at can only be ascertained from experience. In this 

 way the valves and connecting membranes of the diatoms are 

 usually separated and cleaned ready for mounting, which 

 process will be described hereafter. 



Muds will have to be treated in a somewhat different manner 

 from recent gatherings. If the mud is dry, it will have to be 

 broken down by boiling for a few minutes in a solution of 

 caustic potassa, the strength of which must be apportioned to 

 the particular specimen under treatment. After it has been 

 broken down into a soft mud, all of the potash is thoroughly 

 washed off by means of clean water, and replaced by nitric 

 acid, as in the case of recent gatherings. This is boiled, and a 

 little bichromate of potash added as before, and the whole 

 washed. It very seldom happens that the diatoms occurring 

 in mud will be sufficiently cleaned by this process, so that it has 

 to be supplemented by another. The sediment is therefore 

 washed into one of the evaporating dishes and allowed to settle, 

 and as much of the water poured off as possible. Then 

 sulphuric acid, in quantity to a little more than cover them, is 

 poured in, and the vessel gradually and carefully heated. As 

 soon as the liquid shows signs of boiling, bichromate of potash 

 is added, a very little at a time, until the green color first formed 

 by its reaction upon the organic mattter begins to assume a 

 yellowish tint, when no more is dropped in; but a few drops of 

 hydrochloric acid are permitted to fall in, and the liquid is 

 allowed to cool. Of course it will be best if the person under- 

 taking to clean diatoms is somewhat versed in the use of chem- 



