DALY. — ETCH-FIGURES ON AMPHIBOLES. 



385 



agitation. I am inclined to think that convection currents in the warmed 

 acid are sufficient to perform the same function. 



Ill thus fixing on a universal solvent, its temperature, state of concen- 

 tration and of convention, we have narrowed down the variables of the 

 process to one, the duration of attack. This facility in arriving at the 

 conditions of a uniform method of etching is not possible when the caus- 

 tic alkalies are employed and hence these will be referred to only inci- 

 dentally in the following pages. Unless the contrary be stated, the 

 standard conditions of etching are to be understood in every experiment. 



The Optimum Exposure of the Different Species. 



Here the study was confined to cleavage pieces. It was naturally 

 found that as the figures increased in number, they also increased in size, 

 any changes in shape not being sufficient to remove them from the cate- 

 gory of "primary" figures (Becke). In most cases, the general cleav- 

 age surface showed no serious roughening as the figures grew and the 

 attack was allowed to continue nearly to the point where overlapping of 

 the pits (generally aggregated in groups) would occur. This length of 

 exposure usually gave the best figures for study ; hence I have called it 

 the " optimum " duration of attack. Since the determination of the 

 optimum length of immersion was a matter of considerable labor, the 

 result of several trials with almost all varieties, I have thought it worth 

 while to tabulate the results obtained with a certain number of speci- 

 mens. It is to be understood that the following table is only approxi- 

 mately accurate. Perfectly fresh acid was not used in every case, and, 

 of course, the longer the acid remains on the water bath, the weaker it be- 

 comes ; moreover, the amount of steam in the water bath is variable to some 

 extent, and thus the HF might become heated at different rates. Care was 

 taken to allow for such causes of variation from the true optimum. 



Optimum Exposures. 



2J minutes. 



91 " 



2i. 

 1 

 4 



n 

 2 



2i 

 1 



u 



1 " 



1| m. (Sraaragdite). 

 f m. (Richterite). 



