WATER AND VOLCANIC ACTIVITi" DAY AND SHEPHERD. 287 



The sample lb was dipped from the middle of the lava lake on 

 July 23, 1911, by Mr. Frank A. Perret and one of the authors (Shep- 

 herd),^ with the help of a cable and trolley, stretched directly across 

 the center of the pit, and appropriate tackle. The sample la was 

 taken from one of the recent overjflows on the main floor of the 

 Kilauea crater and may perhaps be 15 or 20 years old. The sub- 

 stantial identity of the analyses with each other and with other 

 recent analyses of the lavas of Hawaii" shows that no material 

 change in its composition has taken place in recent years. The most 

 noticeable feature of the new analyses is perhaps the presence of a 

 small amount of molybdenum, which appears not to have been de- 

 tected hitherto. The analyses were most carefully made by Mr. 

 John B. Ferguson, of this laboratory, to whom we take this oppor- 

 tunity to express our thanks. 



THE GASES AND DIFFERENT WAYS OF STUDYING THEM. 



The problem of collecting volcanic gavses which are satisfactory 

 from the chemical viewpoint is a much more difficult matter, as has 

 been already intimated. Hot gases of more or less complicated com- 

 position discharged from an active volcanic vent into the air undergo 

 immediate and violent chemical and temperature changes, the conse- 

 quences of which, with our present limited knowledge of gas relations 

 at these temperatures, can be only partly inferred. It is therefore a 

 matter of the first importance to collect the gases directly from the 

 liquid lava or the explosive vents before contact with the air has given 

 opportunity for these alterations to occur. It may very well be that 

 the physical difficulties attending the collection of volcanic exhala- 

 tions, particularly from volcanoes of the explosive type, will often 

 make it impossible to obtain unaltered magmatic gases for laboratory 

 study, in which case burned gases, or even very dilute mixtures of 

 these Avith air, may prove to be the only products available for study. 

 In this event the student must perforce bow to the necessities of the 

 case. 



Something of the same cautious attitude requires to be maintained 

 toward the study of the flame spectra of burning volcanic gases. The 

 pocket spectroscope is primarily an instrument of preliminary recon- 

 naissance in the field and is sometimes of value, but the pale-blue 

 flames of sulphur and hydrogen are extremely difficult to analyze 

 with the pocket spectroscope, and can not be distinguished at all 

 against a bright background of solid or liquid lava. For this reason 



^This expedition was sent out by the Massachusetts Institute of Technology (Prof. 

 T. A, Jaggar) in the summer of 1911 for the purpose of securing a trustworthy measure- 

 ment of the temperature In the lava lake. The record of the expedition has not been 

 published. 



-R. A. Daly: Magmatic differentiation in Hawaii. Journal of Geology, vol. 19, 1911, 

 p. 305. 



W. T. Brigham ; Loc. cit., p. 33. 



