EXPERIMENT STATION BULLETINS. 491 



From the phjsical standpoint temperature affects many processes 

 in the soil : it causes a movement of the soil moisture and of the salts 

 due to the change in their surface tension and viscosity; it produces a 

 movement of air on account of the change in pressure. A change in 

 temperature also causes a disintegration of the rocks or soil material 

 due to expansion and contraction. 



These biological, chemical, and physical functions of soil tempera- 

 ture emphasize the great importance of the subject and the need for a 

 thorough and extensive knowledge concerning it. Unfortunately our 

 present knowledge of it is very limited. The reasons for this may 

 be divided into two general groups: (1) We do not yet know abso- 

 lutely the nature of heat -and hence of the temperature; and (2) our 

 present methods for measuring soil temperature are very unsatisfactory. 



That our knowledge of the nature of heat is yet uncertain will be 

 at once evident if the two views concerning it are considered. The 

 static view considers heat as being a substance and designates it as 

 caloric. The dynamic view, on the other hand, considers heat as a 

 mode of motion ; or as a measure of the internal energy of the mole- 

 cules of the body. All bodies, it maintains, consist of molecules in a 

 state of vibration and the energy of the motion of these molecules 

 determines the temperature of the body; that a difference of tempera- 

 ture between two bodies merely means that a difffference exists in the 

 energy of their molecules; and that this difference may be equalized 

 with the lapse of time by radiation, conduction, and convection until 

 equilibrium results. This. dynamic theory of heat established by the 

 experiments of Rumford, Davy, and Joule, has been the accepted view 

 of modern physics, while the static theory has been entirely abandoned. 

 Very recently, however, Callender suggests the idea that this old, dis- 

 carded caloric theory may be right after all. He maintains that what 

 we call heat and measure as heat, is merely the energy of heat, the heat 

 itself may well be a substance that carries energy as a stream of water 

 carries the energy that turns the mill-wheel. 



These different views concerning the nature of heat go to show that 

 after all we are not absolutely sure what heat is, of the laws govern- 

 it, etc., even tho modern physics considered it one of its triumphs that 

 it had demonstrated beyond doubt the nature of heat from the dynamic 

 standpoint. 



Undoubtedly by far the greater part of our limited knowledge of 

 soil temperature is due to our unsuitable and unsatisfactory methods or 

 instruments employed for its measurement or study. Until very recent- 

 ly, in fact in all the soil temperature studies that are on record, the 

 mercury thermometers have been used almost exclusively as a means 

 for the measurement of temperature. These instruments, altho in use 

 in almost all scientific temperature investigations, are far from being 

 very satisfactory' for the measurement of soil temperature. One of their 

 serious defects is the unequal length of their mercury bulbs, which 

 gives rise to the difficulty of measuring accurately the temperature of 

 the same depth of different soils. In the second place, if the stems 

 are of the same length and the bulbs are placed at different depths, 

 there will be unequal mercury columns exposed to the atmosphere, and 

 thus, on account of the marked difference of temperature in air and 

 soil, there will undoubtedly be an error introduced in the data. 



