676 SUMMARY OF CURRENT RESEARCHES RELATING TO 



this process is advancing from without inwards, the inner cells also die, 

 and the contraction of their walls is prevented by their connection with 

 the outer layers which have already become stiff; and they also become 

 hard while in the stretched condition. The brittle tissues can be softened 

 by soaking in water for from half an hour to an hour, and do not then again 

 become brittle if again placed in strong alcohol. 



Cleaning" Diatoms.* — Mr. W. A. Terry recommends the following 

 process for cleaning diatoms. No fumes of any consequence are given off, 

 no artificial heat is required, the process takes only a few minutes, and a 

 much larger proportion of the diatoms are uninjured : — 



After washing out the coarse sand and straining out the coarse refuse 

 from the gathering which has not been dried, the material is allowed to 

 settle in the vessel ; the water is then poured off rather closely, so that the 

 amount remaining shall be about equal in weight to the weight of the 

 material dry. Finely powdered bichromate of potash is then added in 

 amount equal to the estimated amount of organic matter in the material 

 exclusive of the sand. It is then stirred until mixed ; for this purpose a 

 glass slip half an inch wide, with rounded edges, is more convenient than 

 a glass rod. Strong commercial sulphuric acid is then dropped in until 

 brisk effervescence is set up, and continued until the acid produces no effect. 

 The whole mixture is then poured into a vessel containing cold water, and 

 after agitation is allowed to settle. The diatoms will now be found to be 

 nearly clean, and only require the usual alkaline treatment and thorough 

 washing. After the addition of the bichromate, the temperature of the 

 material and of the acid should not be less than 70° F. If the diatoms bo 

 not sufficiently cleaned, the operation may be repeated or nitric acid used 

 without much danger. If the material have been dried, it will be well 

 to soak or boil it in water before using acid. Marine muds should be first 

 washed in fresh water to remove the salt, and as they contain more refractory 

 material, the action should be proportionately energetic. Fossil marine 

 earths should be thoroughly softened by long soaking and boiling before 

 being treated with acids, otherwise the gases disengaged would tear and 

 fracture very many of the forms. Boiling in alkalies should be avoided, if 

 possible, as many varieties are softened and distorted by even cold and weak 

 solutions. As first washings, both acid and alkaline, settle very slowly, 

 they should be allowed plenty of time, otherwise the lighter and more 

 delicate varieties would be lost. 



The author states that he usually succeeds in getting the diatoms beaii- 

 tifully white and clean at the first operation, but admits that the process is 

 capable of some improvement. 



Preparing Silver Crystals.f — Mr. F. T. Chapman says that artificially 

 prepared silver crystals make fine opaque objects, either as permanent 

 mounts, or for observing the process of crystallization. They may be 

 readily prepared, although some care is necessary in order to obtain the 

 best results, especially if the preparation is designed to be permanent. 



The deposition of silver from a solution of silver nitrate by means of 

 copper, preferably a copper-wire ring placed in a sufficiently deep cement 

 cell, gives very good results if the wire ring and the thicker mass of crystals 

 at the edge be removed, and the specimen then thoroughly dried and 

 protected by a cover-glass in the usual way. Much better results, however, 

 can be obtained with a brass cell provided with a removable cover or cap 

 (known as the " Pierce cell "), and cemented to a glass slip, the cell being 



* Amer. Mon. Micr. Journ., viii. (1887) pp. 69-71. t Ibid., pp. 99-100. 



