384 PROCEEDINGS OF THE AMERICAN ACADEMY. 



on a water bath, or, again, when immersed several hours in cold HF. 

 Among all these possibilities I have endeavored to secure for all com- 

 parative studies a temperature as nearly constant as possible, and this for 

 two reasons. In the first place, it would not be at all certain without 

 direct proof that the figures with any one reagent remain constant for all 

 temperatures. Bomer discovered that the form and orientation of the 

 figures on quartz produced by attack with HF were affected by the tem- 

 perature of the reaction.* It is reasonable to suppose that temperature 

 may have a corresponding effect on amphibole figures when the same 

 reagent is used. As will be noticed elsewhere, I have been able to deter- 

 mine no sensible variations in figures on (110) from this cause, but it 

 cannot be denied that they are present. Secondly, I wished to tabulate 

 the amphiboles with reference to their power of resisting solution in the 

 etching process. The standard temperature chosen for these reasons is 

 that of the water bath, one that is nearly constant, attained with no dif- 

 ficulty, and found to suit the necessities of the case very well. 



To secure a standard temperature for the reaction repeatedly and expe- 

 ditiously demanded in addition a certain amount of arbitrary treatment, 

 since the amount and initial temperature of the acid have not yet been 

 allowed for. A platinum crucible of the usual slightly conical form 

 and with a diameter at the bottom of about 4 cm. is filled to a depth of 

 1 cm. with the cold acid ; the mineral, resting in a platinum net, is im- 

 mersed, and at the same time the crucible is placed 1 cm. deep in the 

 steam of a water-bath which is kept constantly at 100 degrees Centigrade. 

 The attack is readily checked at any moment by lifting the platinum net 

 and plunging it with the mineral into water. The coating of fluorides 

 could be readily removed by brushing the mineral in running water, or 

 by dissolving them in hot concentrated hydrochloric acid. 



Another point of inquiry in connection with formation of an ideal 

 method would be the effect of increasing the energy of the reaction by 

 the agitation of the acid during attack. Klocke, in his classic research 

 on the alums, found that on agitating the solution in which corrosion pits 

 were forming, the figures grew larger rapidly, due, as he stated, to the 

 dissipat'on of the " Hof " (Losungshof) of liquor near the figure which 

 had become laden with the products of solution. t Experiment of the 

 same kind was carried on with basaltic hornblende, but no material im- 

 provement was effected on the sharpness of figures produced without 



* Neues Jahrbuch, 1891, Beil. Bd. VII. p. 538. 

 t Zeit. fur Kryst., 1877-78, Bd. II. p. 298. 



