[SHAW] IMPROVED METHODS IN HYGROMETRY ot 
It is obviously advisable to protect the paper in these determina- 
tions from dust or from air super-saturated with moisture. In the 
latter case the weight of the paper will be increased, so that it indicates 
a higher vapour pressure than the true one, and if the deposition 
of water is sufficient, drops will collect and fall, removing some of the 
P.0;. 
Although such an instrument as this has, like many others, 
the disadvantage of being empirical and secondary, it appears to have 
the great advantage of being more constant and accurate although 
it is cheap and simple in construction. The vapour pressure can 
be read directly, and if the air is circulating constantly, there is no 
delay in the determination. In case of accident a supply of stand- 
ardized moistened papers can be kept on hand, in order to avoid 
the necessity of-calibration each time that a new one is needed. So 
far none of our moistened papers have deteriorated. 
$ 4. ELECTRICAL METHODS OF HYGROMETRY. 
Two promising electrical methods for the sensitive measurement 
of aqueous vapour pressure were developed. They are:—(1) To 
measure the electrical resistance of a hygroscopic insulator such as 
paraffin wax, when a convenient maximum amount of surface per 
unit volume is exposed; (2) to measure the electrical resistance of 
one of the ‘‘moistened papers’’ arranged as described in section 3. 
Rapid and large variations resulting in errors of as much as four 
or five per cent, which are due chiefly to polarization and to lack 
of regularity in the flow of air, have not yet been eliminated. The 
matter seems worth recording, however, because when employed 
merely as hygroscopes, these arrangements have shown a surprising 
sensitiveness, changes in vapour pressure of less than = of 1% being 
detected with the aid of a sensitive galvanometer. 
§ 5. REMARKS ON THE TERM “RELATIVE HUMIDITY.” 
The term ‘relative humidity’ has been given considerable 
prominence by many observers because it indicates directly the 
fraction of saturation. The dryness or dampness of a sample of air 
depends on the relative humidity and not upon either the absolute 
humidity, or the aqueous vapour pressure alone; but the importance 
of this expression has perhaps been over emphasized. If needed, 
it can always be obtained from the vapour pressure (or from the 
absolute humidity) and the temperature by means of the usual tables. 
As it is a function of both the absolute humidity and the temperature, 
