52 



SCIENCE-GOSSIP. 



[March 1, 1865. 



CLEANING DIATOMACE^. 

 By a. J. Roberts. 



TT is often a source of diflSculty to a micro- 

 ._ scopist who for the first time attempts 

 the study of the various forms of Diatomacece 

 contained in infusorial eai-ths and guano to 

 entirely free thein from foreign matters. In 

 order to do this some knowledge of chemistry 

 is requisite, more, indeed, than is often pos- 

 sessed by microscopists ; it is, therefore, the 

 object of the writer in the following remarks 

 to supply this to a certain extent by pointing 

 out some of the constituents of these deposits 

 and guanos, which are the principal sources 

 of our foreign and some of our fossil Diato- 

 macece. 



The shells of the Diatomacece, it is well 

 known, ai"e composed of silex, or, in common 

 parlance, flint, and are consequently not acted 

 on by any of the ordinary mineral acids in 

 common use, viz., the nitric, hydrochloric, or 

 sulphuric ; but, on the other hand, boiling 

 in solution of the caustic alkalies, soda and 

 potash, will dissolve them. It is also to be 

 observed, that they are unalterable by the 

 action of a red heat, but if heated with either 

 soda or potash, tlie silex combines with the 

 alkali, and fusion, and consequent loss, takes 

 place. 



These facts are well to be noted, that the 

 operator may know what chemical agents are 

 to be avoided in working on substances con- 

 taining these Diinute organisms. 



With regard to the constituents of the 

 earths in which the shells are found, these 

 consist principally of lime in various states of 

 combination, usually either carbonate or sul- 

 phate, or perhaps both, frequently coloured 

 with oxide of iron, and also silex in the form 

 of sand, which latter can only be separated 

 by careful fractional washing. 



The carbonate of lime is soluble -with eficr- 

 vescence in nitric or hydrochloric acid ; either 

 of these will form a soluble salt which may 

 easily be removed by washing. Sulphuric 

 acid should be avoided, for it forms with the 

 lime a sparingly soluble sulphate. Sulpliate 

 of lime, if present as a constituent, will proba- 

 l:i]y not be in very large quantity, and may be 

 removed by frequent washing, as it is soluble 

 to a certain extent in v/ater. The oxide of 

 iron, also small in quantity, may be dissolved 

 by the nitric or hydrochloric acid, and should 

 these fail when used separately (which Avill 

 sometimes be the case from the peculiar state 

 of combination in v.hich the iron exists), they 

 may be employed mixed together, forming 



aqtca regia. The process of cleaning may be 

 known to be completed when the deposit is 

 quite white and acid have no further action 

 on it j a small portion should then be exam- 

 ined (under water) by the microscope, and if 

 quite clean should be well washed, first with 

 common water-, and finally with distilled 

 water. The latter is indispensable to com- 

 plete the operation, that all traces of acid and 

 soluble salts may be removed. 



If the earth under examination occurs in 

 tolerable sized masses it should be br-oken into 

 fragments about as large as a small pea, and 

 then dropped gradually into a Florence flask 

 or open evaporating basin, about half full of 

 nitric or hydrochloric acid, diluted v/ith one- 

 third of its volume of distilled water. It is 

 better to drop the substance -under operation 

 into the acid than to pour the acids on the 

 material, as the action takes place more gra- 

 dually and v-fith less frothing than is generally 

 the case when much carbonate is present, 

 which sometimes causes the liquid to escd])e 

 from the vessels, and consequent loss. When 

 the acid ceases to act, heat may be applied by 

 any suitable means, and v/hen all action ceases 

 the mixture should be allowed to i-est, that 

 the deposit may subside, and the liquid 

 poured off as close as possible, and replaced 

 by strong acid, and the acid again aided by 

 gentle heat. Should the deposit not be quite 

 white the mixed acids must be employed, and 

 the process finished by washing with distilled 

 water. 



To j)roceed to guanos ; these present more 

 difficulty, their composition being much more 

 complex, and containing, besides the lime salts 

 and oxide of iron above mentioned, salts of 

 soda, potash, and ammonia ; also much animal 

 matter, and some phosphate of lime. The ^ 

 salts of the alkalies are soluble in water, and 

 the guano being usually in a more or less dis- 

 integrated state, it should be first well washed 

 with boiling water, either on a paper filter or 

 by repeated affusions, until the liquid comes 

 off tasteless. Boiling water is pi-eferable to 

 cold, for the heat expels air-bubbles and causes 

 the deposit to settle down into a smaller S2)ace; 

 then the deposit must be subjected to the 

 action of the acids as directed for the prepara- 

 tion of earths, to get rid of the lime salts. The 

 partially cleaned deposit, which is now much 

 smaller in bulk, must be se})arated as much as 

 possible from the liquid, and strong sulphuric 

 acid, in sufficient quantity to cover it, poured on 

 to about the depth of half an inch, heat applied 

 and continued for some time, but the liquid 

 must not be made to boil ; the result will be 

 au almost immediate blackening of the liquid, 



V •■*?■'' 



