OLFACTORY HUNTING RESPONSES OP THE DOG 77 



diffusion could be represented by one of the well-known " curves 

 of decay." The absolute intensity of the stimulus (i.e., the 

 amount of odorous substance present in the track at a given 

 moment of time) may, within limits, be formulized : 



Log S t = Log S — kt, 

 wherein S equals the amount of the substance first deposited, 

 t the time which had elapsed since the deposit was made, and k a 

 constant function dependent on conditions of temperature, 

 pressure, etc. 



In the case under consideration the stimulus-intensity at the 

 track a is nearly zero when it is presented to the dog. The 

 absolute difference of stimulus-intensity at a, b and c would have 

 to be extremely small, since the difference in the respective 

 values of t is of the order of one part in nearly 11,000. Further: 

 even this difference between a and b would exist only if they were 

 simultaneously presented. Since the dog is supposed to be 

 following the trail of the rabbit, for him to be affected by even a 

 part of the difference between a and b it is necessary that he 

 travel faster than did the rabbit in making the tracks. If the 

 dog travels at the same rate as did the rabbit, when he reaches b 

 its intensity will be just equal to that of a when a was passed. 

 Moreover, in actual practice other difficulties arise. Suppose 

 the rabbit has run from moist ground to dry ground. The smell- 

 substances are diffused more rapidly under conditions of relatively 

 small humidity that under conditions of greater humidity. 

 The stimulus-intensity of the recently made tracks on dry ground 

 could thus be less than those made earlier on the wet ground. 

 In such case our assumption fails to explain the dog's failure to 

 show confusion. 



But the dissipation of the smell-substance may be a complex 

 process. For instance, it may be deposited, not in a gaseous 

 state, but as a liquid or solid. In such case vaporization must 

 precede diffusion. Vaporization, conditions being constant, 

 proceeds at nearly a uniform rate in the open air. The amount 

 of substance present in a gaseous state might thus be as great 

 at a very advanced stage of dissipation as at an earlier stage. 

 Since the substance to be odorous must be gaseous, we are not 

 warranted in assuming that the stimulus-intensity is greater at 

 a recently made track than at one made earlier, unless we know 

 that all the smell-substance in the later track has been vaporized. 



