SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 53 



winters it is difficult to keep even a fairly well-heated room constantly at as high a 

 temperature as 19 "5°. 



We see then that, although the temperature 19 "5° 0. may be very suitable and 

 be very easily maintained during the greater part of the year, this may not be the case 

 under the extreme conditions of raid-winter and mid-summer. It is therefore necessary, 

 besides the particular temperature selected as the basis of the mean temperature of 

 the locality, to have a higher temperature to meet the case of a warm summer, and a 

 lower temperature to meet that of a cold winter. When the heat outside is such that 

 the temperature of the air of the laboratory naturally rises above 19 "5°, I use 23° C. 

 as the particular temperature at which all my observations are made. On the rare 

 occasions when this temperature is too low, I use 26° C, and this is a very useful 

 temperature in tropical countries. At sea, even in equatorial regions, the highest 

 particular temperature that would be required is 30° C. In winter the temperature 

 of a laboratory or other inhabited room should never fall below 12° C. In these 

 circumstances 15° C. has been adopted as the particular temperature. 



The room used as laboratory should be of moderate dimensions, rather small than 

 large, because it must be occupied only by the experimenter, and he must have absolute 

 control over it. It should be illuminated by sky light, and the direct rays of the sun 

 must be absolutely excluded. In our latitudes this means that the window must have 

 a northern exposure. The room should be furnished with central heating, preferably 

 by hot water. 



When the particular temperature required for the liquid, the specific gravity of 

 which is to be determined, is 19 '5° C, the temperature of the air should be maintained 

 at 19*0° to 19 "3°, but this depends on the room. When the temperature of the liquid 

 has been brought exactly to 19 '5°, it will remain constant at this temperature when the 

 temperature of the air in the neighbourhood of the cylinder is at 19 '2° or 19 "3°, because 

 the heat which is removed from it by convection is equal to that supplied to it by 

 radiation, principally from the experimenter himself. 



The constant temperature of the liquid in the cylinder is the integral efi"ect of a 

 number of separate elements, of which the principal are the temperature of the air 

 within and without the room and that of the experimenter. In mild weather, when 

 the temperature of the air outside is about 14° to 17° C, it is generally very easy to 

 regulate the temperature of the air in the laboratory so that that of the liquid in the 

 cylinder may remain constantly at 19*5° for the duration of the experiment. In cold 

 weather, however, when the temperature of the air outside may be 8° or 10° or even 

 more below the temperature of the room, the liquid is apt to experience sensible cooling 

 by radiation to the outside through the glass of the window. (Of course the window 

 must always be shut when experiments are being made, because the slightest 

 draught striking the exposed stem of the hydrometer disturbs the reading as well as 

 the temperature.) In these circumstances it is often difficult to maintain its tempera- 

 ture at 15° for the duration of the experiment. 



