Physical Problems in Study of Conditions for Health 29 



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 the second jar and recording successively the equilibrium weights 

 of each specimen, to determine the variation of the vapour pressure 

 with temperature in such a manner that the usual errors are either 

 eliminated or readily determined. 



The method of calibration has been successfully applied by 

 the writer in the case of thin films of sulphuric acid, thin paper 

 containing diluted calcium chloride, and thin paper containing 

 diluted phosphorous pentoxide. 



(2) The variation of the internal temperature of small volumes 

 of hygroscopic materials, with difference in vapour pressure between 

 its surface and the surrounding air, should be investigated. This 

 is important because in many cases there is a surprising temperature 

 variation during a very short interval of time after a sudden change 

 in external vapour pressure has been produced. The data may also 

 be utilized for purposes of hygrometrical determinations in small 

 confined spaces. 



A thermo-couple method utilizing the type of hygrometer 

 described above, should lead to definite results. This variation 

 has been observed qualitatively in the case of bundles of hair, 

 tobacco, fine shavings of wood, and small piles of powdered salts, 

 but no systematic investigation has apparently been attempted. 



(3) The influence of the average irregularity of a hygroscopic 

 surface, on its vapour pressure requires investigation. This efifect is 

 appreciable in the case of hard surfaces which may be found either 

 smooth in one instance, or covered with sharply peaked irregularities 

 in another. 



A series of observations with the microscope on the rate of 

 evaporation of curved surfaces of hygroscopic solutions, of known 

 concentration and of various curvatures, could be obtained under 

 known conditions at the end of capillary tubes for the purpose of 

 comparison with the elementary theory of the influence of curvature 

 on vapour pressure. A record of the variations in vapour pressure 

 from coarse surfaces which absorbed the same solution, could 

 then be interpreted. Great care is necessary in all experiments of 

 this type to eliminate or to control the effects of incident radiation, 

 and irregular convection. 



(4) With most systems of ventilation "convection" heating in 

 buildings leads to a condition where the air is warmer than most of 

 the surrounding objects, in "radiation" heating the converse is 

 usually the case. As the latter condition is probably the more 

 healthy, and as the former condition is the more prevalent in this 

 country, there is obviously need for development. 



