brun's hygrometric observations 579 



tion to the portion disappearing as vapor in the unsaturated atmosphere, 6 

 a considerable additional quantity will condense about the finely divided 

 sulphur particles, serving as nuclei of condensation. 



Furthermore, in our opinion, Bran'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 thmselves. The very 

 care exercised by Bran 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 compara- 

 tive dryness of the cloud would also find ready explanation. 



Brun's final contention (section 6, p. 577) 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 appears 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 Volcanique," this 

 is the point which Brun himself regarded as the most convincing observa- 

 tion of all. It appears to be a matter of grave doubt whether the read- 

 ings of a dew-point hygrometer in an atmosphere containing S0 2 and 

 S0 3 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. Fur- 

 thermore, the effect on the dew-point apparatus itself of exposure to the 

 cloud containing S0 2 and S0 3 may be a factor of considerable signifi- 

 cance. The first reading of the instrument might well be approximately 

 correct, but subsequent readings would surely all be subject to the effect 

 of uncertain amounts of S0 2 and S0 3 carried by the instrument in con- 

 sequence of the first exposure. This would have the effect of rendering 

 all the subsequent readings of the series quite valueless as a measure of 

 the water content either of the air or of the cloud. 



In order to support the view that the atmosphere within the cloud con- 

 taining S0 2 and S0 3 is necessarily drier than air which does not contain 

 these substances, several measurements of dew-pointa were made by us in 

 an appropriate laboratory apparatus, of which the results will be found 



Prof. J. P. Iddlnge, Prof. H. D. Glbbs, of th<> University of Manila, and several chem- 

 lsts from the Philippine Bureau of Science have observed gaseous emanations rising in 

 great volume near the volcano Taal, which were Pound to contain Largs Quantities of 

 water and yet gave n<> tfacs of i cloud, (Unpublished records of tiu> Bureau of Science, 

 Manila, P. I.) 



