NUCLEAR CONDENSATION OF CERTAIN ORGANIC VAPOURS. 
469 
connected with the phenomena that the least expansion necessary to catch the nuclei 
produced in air and acetic acid vapour depends on the intensity of the Rontgen rays. 
Thus the least expansion for condensation in this acid with intense X-rays was found 
to be (see p. 454) U441, and with feeble rays R471. Now the radiation which 
produces the natural ionisation in air is largely due to a penetrating radiation* of very 
small intensity. It will be necessary to make a special investigation to clear up the 
question of the relative efficiency of the natural nuclei and the Rontgen-ray ionic 
nuclei in case of formic, acetic, and butyric acids. 
Professor BarusI observed, in 1906, that the intensity of the Rontgen rays similarly 
affects condensation in air and water vapour. 
In the table on p. 470 the least supersaturations which will produce condensation 
on the natural nuclei have been calculated, y = l - 404 has been used in each case. 
The values of tt 2 used in this paper have been inter- and extra-polated by the 
method of Ramsay and Young, the Kirchhoff-Rankine formula, and graphically. 
7 t 2 of each substance is for a lower temperature in the table on p. 470 than in the case 
of the table on p. 460, and consequently the above values of the supersaturation are 
less accurate than those given in the latter table. For this reason the relation of S 
to chemical constitution is discussed for condensation in air ionised by Rontgen rays. 
It will be seen, however, that the supersaturation necessary for condensation on 
natural nuclei is related in essentially the same way to the chemical constitution of 
the vapour as in the case already discussed. 
Sign of the Charge of the more Efficient Nucleus. 
It has been mentioned how C. T. R. Wilson found that a smaller supersaturation 
gave condensation on the negative ion than on the positive in air initially saturated 
with water, or, in other words, how he found the negative ion to be the more efficient 
nucleus for water vapour. 
The chief difficulty encountered in finding the more efficient ion for organic vapours 
was to make an expansion chamber of material which is unacted upon by them. 
Sodium silicate, lead oxide and glycerine, sealing wax, rubber dissolved in benzine, 
were tried as cements for the joint between the aluminium and glass in the apparatus 
described on p. 449 ; the two first, which were not acted upon by esters, failed to 
make a permanently gas-tight joint. Sealing wax and rubber are acted upon by 
many organic liquids. If, however, the vapour only of the ester (say) is allowed to 
come in contact with the rubber, the action is very slow and the rubber appears 
unaffected. A sensitive method of detecting the action of the ester on the rubber is 
to determine the expansion necessary to catch X-ray nuclei immediately after filling 
the apparatus with the ester, and then again twenty-four hours or so later; a change 
* McLennan, ‘Phys. Rev.,’ 16, p. 184 (1903); Rutherford and Cooke, ibid., 16, p. 183 (1903). 
f ‘Physical Rev.,’ 22, 2, February, 1906, 
