IX THE IMIESENCE OF DUST-FREE AIR AXD OTHER GASES. 
;^oi 
been used, not that over a flat surface. But for drops of 5 X 10“® centim. in radius 
the difference, as calculated hy Lord Kelvin’s formula, does not amount to more than 
1 or 2 per cent., and is negligible for the present purpose. 
The exact theory of the colour phenomena which are produced by clouds of small 
water drops such as are formed in these expansion experiments, has not, as far as I 
am aware, been wmrked out. This is especially true of the colours filling the centre 
of the field within the diffraction rings. Since the whole of the colour phenomena 
from the first appearance of small diffraction rings to the disappearance of all colour, 
except the bluish or greenish-white, are confined within quite a narrow range of 
expansions, the size of the drops ev^idently diminishes with great rapidity with 
increasing expansion. 
When all diffraction colours disappear, and the fog appears white from all [)oints of 
view, as it does when amounts to about 1’44, we cannot be far wrong in assuming 
that the diameter of the drops does not exceed one wave-length in the brightest part 
of the spectrum, that is, about 5 X 10~® centim. That the absence of colour is not 
due to the inequality of the drops is evident from the fact that the colours are at 
their brightest when vjv^ is only slightly less than 1'44, and from the perfect regu¬ 
larity of the colour changes up to this point. 
Taking the diameter of the drops as 5 X 10~^ cub. centim., we obtain for the 
volume of each drop about 6 X 10“^^ cub. centim., or its mass is 6 X 10“^^ grin. 
Now, we have seen that when the expansion is such as produces the sensitive tint 
(when t’.j/iq = 1’42), the quantity of water which separates out is about 7’6 x f 0“® grm. 
in each culiic centimetre. With greater expansions rather more must separate out. 
We, therefore, obtain as an inferior limit to the nuniher of drops, w hen Vo/<q = 1‘44, 
per cubic centimetre. 
T't) X Hr'’ 
G X 10-1^ 
Ejj'act of the lidittgeii Rags on Condensation. 
A statement of the results obtained when moist air is subjected to the action of the 
llontgen ra 3 ^s, and then allowed to expand, has already been published,* but tlie 
experimental details were not given. The experiments were made with the second 
form of apparatus (fig. 2). A bulb which was giving out the X-rays energetically, as 
tested by a fluorescent screen, wms fixed about 10 centims. from A. It was found 
that if expansion was made when the bulb was in action, or within a second or two 
after swutching off the current from the induction coil, the number of drops produced 
Avas greatly increased if the expansion was suoh as would have caused rain-like con¬ 
densation in the absence of the rays. Instead of a show'cr settling in one or two 
* • Proc. iioy. vol. GG, [i. .’IGS, ISGG. 
