172 Proceedings of Royal Society of Edinburgh. [sess. 
When the plan shown on fig. 1 is adopted, it may be advisable to 
lengthen the part E, so as to bring the lower end of the stopcock S 
in a line with the top of the tripod, and to bind the apparatus 
together by means of a support attached to S, and fixed to the tripod 
by the nut and screw N. 
W 7 hen working this instrument daylight is used for illuminating 
the stage, and is found to work well in the open air. A good deal 
of the glass top of the receiver is left uncovered with blotting- 
paper to let in as much light as possible ; and a magnifying glass, 
not shown in drawing, is used for counting the drops on the stage. 
This glass should have as little brass mounting as possible below 
the lens, so as not to interfere with the illumination of the stage. 
When working in pure air, the measuring apparatus M is not neces- 
sary, and may be removed, and the filter screwed direct into the 
stopcock S. 
The whole apparatus weighs with tripod-head only 3 lb. 0 \ oz. 
and is packed into a tin-lined leather case 8x5x3 inches. The 
apparatus with case weighs a little over 5J lbs., but this might be 
reduced by omitting the metal case. It is, however, easily carried 
by means of a shoulder strap. The legs of the tripod weigh only 
15 oz., and when fitted together form a round staff, which makes a 
good walking stick when provided with top and bottom caps of 
indiarubber.* 
The Prolonged Action of Sea-Water on Pure Natural 
Magnesium Silicates. By Alexander Johnstone, 
F.G.S. 
(Read February 4, 1889.) 
Pure mineral magnesium silicates are amongst the most difficult 
substances to decompose by naturally occurring agents. Pure water 
exerts no chemical action on them, neither does water containing 
carbonic acid gas,f even although the latter body be present to the 
* After a considerable experience with this apparatus, I have never found 
it necessary to use the smallest of the measures, even when testing the air of 
cities. It may, therefore, be omitted. But if retained for special reasons, 
the centre stopcock should be placed at an angle with the others, so that 
the handles may be more easily worked, than when they are all crowded in 
one line. 
t As far as I have been able to ascertain by experiment, carbonic acid water 
cannot decompose a 'pure magnesium silicate, such as white talc. 
