21 199 



heat. Nearest to the springs tlie remnants of vegetation were (|uite black, farther 

 away they were whitish, and finally yellowish, as a transition to the natural green 

 colour. On the whole the terrestrial heat is probably on the increase in the hot 

 springs here. This is confirmed, amongst other things, by the fact that in the 

 group to the extreme north-west, there is a large mud volcano, wliich appears to 

 have come recently into existence. This mud volcano (Plate XII) is about 5 m. in 

 diameter, and we estimated its depth, reckoned from the edge of the spring down 

 to the surface of the mud, at 2 m. The mud is continually thrown up to the 

 level of the edge of the basin, that is, about 2 m. high. About 10 or 12 m. south 

 of the mud volcano, we saw a peculiar little spring which also appears to be quite 

 recent. As the illustration shows (Plate XIII) the spring has piled up the thick 

 mud around its mouth, and in that way a sharp-pointed cone has been raised; this 

 is on account of the mud indurating as soon as it is thrown from the outlet of the 

 spring, and therefore obtaining sufficient solidity to stand upright in a conical form. 



II. 



The methods employed in the experimental work. 



1. Testing the Radioactivity. 



The investigation of the radioactivity of the hot springs comes under two 

 heads, viz. : testing the radioactive emanations contained in the gases from the 

 springs, and the examination of the sediment and mud taken from them, with a 

 view to determining the radioactive substances they may contain. 



The apparatus which I have employed to gauge the radioactive emanation, 

 is reproduced in the accompanying outline drawing, (Fig. 5). The apparatus con- 

 sists of four parts: the electroscope (E), the ionisation chamber (/), the battery (B), 

 and a reading microscope (not shown in the illustration). 



The electroscope (/i) differs from an Elster Geitel electrometer in that it has 

 only one aluminium leaf, and that the electroscope vessel is entirely of brass. In 

 order to be able to observe the leaf, the electroscope vessel is provided with two 

 circular windows, 15 mm. in diameter. The window in the front of the vessel is 

 closed by a plane sheet of glass, through which the charged leaf may be observed, 

 while the leaf receives the light necessary to the observation through the window- 

 on the back side of the vessel, this window consisting of a sheet of opaque glass. 

 The rod (g), which holds the aluminium leaf, passes airtight through the amber cork, 

 and its lower end is made to screw on to a brass cylinder (A) 12 cm. high and 

 2 cm. in diameter. This cylinder is enclosed in the ionisation chamber, — a sheet 



