RESEARCH METHODS IX STUDY OF FOREST ENVIRONMENT. 107 



the highest concentration of the cell sap, he was led to investigate 

 this matter further, Finding nowhere any reference to experiments 

 on the latent heat of vaporization of solutions, and believing that the 

 conception of the fixed nature of that quantity for water was based 

 upon the fact that the condensation of steam had always been em- 

 ployed to determine it, he has been led to perform a number of ex- 

 periments with solutions and with distilled water. 



The most important and convincing of these shows that at the 

 respective boiling points of water, and various solutions up to the 

 point of saturation (for sodium chloride), the latent heat of vapori- 

 zation, determined directly by means of an electric heating element, 

 is practically a constant, though perhaps varying inversely as the 

 absolute boiling point. Thus a saturated salt solution whose boiling 

 point is 7° above that of water and whose osmotic pressure is theo- 

 retically about 400 atmospheres, requires only 4 per cent less heat, 

 per unit of water evaporated, than does pure water. This, however, 

 does not solve the problem, as will be seen from the fact that when 

 placed over a steam bath the saturated salt solution evaporates at 

 a rate of less than 5 per cent of that for pure water. There is in the 

 problem, therefore, very evidently some factor besides vapor pres- 

 sures and latent heats of vaporization when an external supply of 

 heat is concerned. It appears to be a matter of conductivity and 

 possibly also of convection. Further investigation of the problem 

 is urgently needed. 



Method of determining freezing -points. — Since, as has been stated, 



the treatment of the leaves of forest trees, especially conifers, is 



likely to present some complications because of the extreme dryness 



which they sometimes show, it is believed the whole-tissue method 



of McCool and Millar (131) is likely to be ineffective. Hibbard and 



Harrington (126) are therefore quoted here on the process used by 



them and involving grinding of the frozen tissues. From this basis 



any investigator will certainly be able to devise modifications to suit 



his special conditions. 



The apparatus used in our tests was the Beckmann outfit ordinarily used for such 

 work and described in books on physical chemistry, consisting of a Beckmann ther- 

 mometer, freezing tube, outer jacket, and a battery jar containing the freezing mixture. 

 The freezing point of distilled water was taken as zero, and the lowering of the freezing 

 point of the pulp was obtained by subtraction. When determining the freezing point 

 of distilled water an electric stirring device was used consisting of battery, metronome, 

 magnet, and platinum stirrer, but this was not employed in determinations made upon 

 pulps. The pulp was allowed to undercool about 1°, after which the beginning of 

 solidification was brought about by rotating the thermometer backward and forward 

 a few times in the pulp. When the undercooled mass of pulp was thus disturbed 

 the temperature began to rise almost immediately and soon came to rest, after which 

 the thermometer was tapped several times and the final reading then taken. This 



