280 aisinual keport smithsonian institu'lion, 1913. 

 brun's hygrometric observations. 



The remainder of Brun's observations of the apparent absence of 

 water vapor may find appropriate explanation in the fact that they 

 were made in an unsaturated atmosphere (as shown by his elaborate 

 records of the hygroscopic state of the air during his observations) 

 at a distance of more than 250 feet from the point of emergence of 

 the gases, and the further fact that the cloud not only carries sulphur, 

 but two of its oxidation products, SO^ and SO3, both of which in 

 these circumstances are effective drying agents. It may very well 

 happen that Avater is given off in considerable amount by the volcano 

 and yet remains invisible; for, in addition to the portion disappear- 

 ing as vapor in the unsaturated atmosphere,^ a considerable addi- 

 tional quantity will condense about the finely divided sulphur par- 

 ticles, serving as nuclei of condensation. 



Furthermore, in our opinion, Brun's explanation of what he deemed 

 to be crystals of hydrated salts in his vacuum tubes and in the pipe 

 line through which his gases were pumped is a somewhat fortuitous 

 one, and certainly leaves an element of reasonable doubt whether 

 their presence was entirely due to moisture carried by the tubes them- 

 selves. The very care exercised by Brun would seem to make this 

 unlikely except for the fact that it was offered by Brun himself. If 

 it could be shown that these hydrous salts were regular inhabitants 

 of the sulphur cloud, the comparative dryness of the cloud would 

 also find ready explanation. 



Brun's final contention (sec. 6, p. 2T8) that a dew-point hygrometer 

 carried along the rim of the crater shows a lower humidity within the 

 cloud than in the clear air immediately outside of it appeal's to be 

 open to serious criticism from the physical side, although if one may 

 judge by the space given to these observations in " L'Exhalaison Vol- 

 canique," this is the point which Brun himself regarded as the most 

 convincing observation of all. It appears to be a matter of grave 

 doubt wdiether the readings of a dew-point hygrometer in an atmos- 

 phere containing SOg and SO3 have any significance whatsoever, in 

 view of the well-known affinity of these compounds for water. The 

 cloud could hardly be charged with better drying agents than these 

 under the conditions described; it might, therefore, a priori, be 

 expected to contain less free moisture than the adjacent atmosphere 

 which does not contain these drying agents. Furthermore, the effect 

 on the dew-point apparatus itself of exposure to the cloud contain- 



1 Prof. J. P. Iddings, Prof. H. D. Gibbs, of the University of Manila, and several chem- 

 ists from the Philippino Bureau of Science have observed gaseous emanations rising in 

 great volume near the volcano Taal, which were found to contain large quantities of 

 water and yet gave no trace of a cloud. (Unpublished records of the Bureau of Science, 

 Manila, P. I.) 



