130 CARNEGIE INSTITUTION OF WASHINGTON. 



are much needed for the synthetic work, and, although the field of 

 possibilities must remain somewhat restricted here, when compared 

 with more soluble substances, nevertheless the sulphides possess con- 

 siderable advantage over the sihcates in this particular. Finally, for 

 a reasonably complete knowledge of the sulphides the redetermination 

 of such physical constants as the specific gravity and crystal angles 

 on pure synthetic or exceptionally fine natural material is much 

 needed. 



The direct investigation of secondary sulphide enrichment; ^. e., 

 the precipitation of heavy metals from solution by various sulphides, 

 has hitherto been confined chiefly to qualitative tests. In the fight 

 of a fuller physical and chemical knowledge of the mineral sulphides, 

 these processes should now be studied quantitatively. 



A strange notion prevails that in work of this character the impor- 

 tant thing is to imitate nature's complexity. Even very recent 

 numbers of scientific journals contain work in which this principle 

 is still followed. The exact opposite is of course the practicable 

 method. The action of the simple and pure sulphides must first 

 be tried separately with each of the principal constituents which are 

 found in the natural solutions. 



There is also a tendency among students in this field to confine 

 themselves strictly to natural conditions, even in respect to temper- 

 ature and time. This is obviously quite impracticable also, except 

 perhaps for those favored amateurs who are not subject to the usual 

 limitations of time and economic considerations. A more successful 

 and much more expeditious plan is to raise the temperature to a 

 point where the reaction to be studied attains a measurable rate, and 

 having ascertained its nature, to find how it is affected by changes 

 of temperature. 



In the study of these reactions with the sulphides, mixtures are 

 often obtained which are quite difficult to deal with. It is then of 

 the first importance to find what and how much goes into solution 

 and what and how much is precipitated. It is perhaps too much to 

 expect that all the alterations which the sulphides of copper appear 

 to undergo in nature during the processes of secondary enrichment 

 can be duplicated in the laboratory within a limited time, but active 

 work is now in progress all along the line, and enough has been 

 accomplished already to assure a good measure of success. 



One feature of this undertaking is novel. It is one of the very 

 few instances where geologists in the field have cooperated directly 

 with physical-chemists in the laboratory on a geologic problem. To 

 the chemist the opportunity to get material and data from a wide 

 area and to have chemical predictions tested by field observations is 

 quite unusual, and the possibility of mutual suggestion between the 

 field and the laboratory worker is full of promise. 



