MR. W. N. SHAW ON HYGROMETRIC METHODS. 77 



drying tubes C. Hence observations made with the instrument may quite fairly be 

 taken to be observations upon air whose hygrometric state is really accurately known ; 

 the degree of accuracy will be clear from the particulars of the experiments. The 

 experiments accordingly group themselves in the following manner : 



I. Experiments to ascertain the limits of accuracy of agreement between the 

 method of saturation and the chemical method. 



These experiments are practically a repetition of REGNAULT'S work with my 

 arrangement of the apparatus and absorbent substances. This repetition is necessary 

 (1) as a preliminary, in order to make sure that the apparatus is in satisfactory 

 working order, and as a test of the drying substances used (Table II.) ; (2) because 

 UEGNAULT'B observations were undertaken with the view of determining the density 

 of vapour in saturated air, and nearly always the air was practically saturated when 

 it reached the drying tubes. In order to complete our knowledge, we have to extend 

 the observations to cases in which the temperature of the air when it reaches the 

 tubes is considerably above the temperature of the saturator. In this case one would 

 expect a priori that the agreement would be very nearly the same, but I know of no 

 published experiments to directly test the point. 



II. Experiments to determine whether the interposition of the vessel C interferes 

 with the concordance of the results obtained. The only apparent reasons for such 

 interference are condensation upon the connecting tube or the vessel C, or leakage 

 from the dew-point instrument. The second part of I. may, therefore, be taken with 

 these. The results of I. and II. are shown in Tables II. to V. 



III. Comparisons of the results of the dew-point instrument with those of the 

 saturator and chemical method when all three are taken together. (Table VIII.) 



IV. Observations with the dew-point instrument compared with the results of the 

 saturator only. A few such observations were made for the purpose of testing special 

 points under circumstances that made the double testing of the air inconvenient or 

 undesirable. (Exp. 73 to 83.) 



I may now proceed to give the details of the experiments. 



THE METHOD OF SATURATION AND THE CHEMICAL METHOD. TESTS OF THE 

 EFFICIENCY OF THE APPARATUS AND OF THE DESICCATING SUBSTANCES. 



1. The method of saturation which is here referred to simply means passing air 

 through some form of apparatus by which it is saturated, the temperature of satura- 

 tion being read by a thermometer placed in the saturator. This is, of course, not a 

 method of measuring the pressure of moisture in a given specimen of air, but simply a 

 means of obtaining air the pressure of vapour in which is known from its temperature. 

 The saturators used will he described later. 



2. The chemical method consists, as is well known, in causing a known volume, v, 

 of air to pass through weighed tubes capable of abstracting the whole of the moisture 



