1864.] 



Prof. Guthrie on Drops. 



451 



It was found impossible to arrest an exact number of drops when the 

 rate was faster than 60 drops in 25". A few rather discordant results, got 

 at the rate of 60 drops in 20", gave a mean of 0*09264 grm. as the weight 

 of a single drop ; this tends to show that at this high rate the drops were 

 considerably larger than at any lower rate. 



Towards the end of the Table, at the slower rates, the error of time be- 

 comes so exaggerated (the least alteration in the adjustment of the instru- 

 ment makes so sensible a change in the entire time-lapse) that it is nearly 

 impossible to avoid an error of about 0"'5 in the whole time of several 

 minutes. Although the time-error thus becomes palpable, it nevertheless 

 remains, relatively to the whole time-lapse, as immaterially small as the 

 inappreciable errors of the swifter rates of dropping. 



The numbers of Table lY. present us with several interesting and im- 

 portant facts. 



From fft='333 to (/t= '433 there is diminution. 

 „ ='433 „ „ = '450 increase. 

 „ ='450 „ = "467 „ diminution. 

 „ „ = '467 = '500 increase. 

 „ ,j ='500 =12*000 continual diminution. 



The most prominent fact is that, o)i the ivhole, the drops undergo a 

 continuous diminution in weight or size as fft increases. To such an 

 extent is this the case, that the most rapidly falling drops of the above 

 Table are nearly twice as heavy as the most slowly falling ones. The cause 

 of this is probably to be sought for in the circumstance that when the 

 flowing to the solid is more slow, the latter is covered with a thinner film 

 of liquid, so that, as the drop parts, the solid reclaims by adhesion more 

 of the root of the drop than is the case when the adhesion of the solid to 

 the liquid can satisfy itself from the thicker j&lm which surrounds the drop 

 in the case of a more rapid flow. The influence of rate is seen to extend 

 even to the exceedingly slow rate of (/t=l2". 



This connexion between rate and weight (or quantity) should not be lost 

 sight of by prescribers and dispensers of medicine. A pharmacist who 

 administers 100 drops of a liquid drug at the rate of three drops per 

 second, may give half as much again, as one who measures the same 

 number at the rate of one drop in two seconds, and so on. 



For our present purpose the effect of rate upon the size of a drop is of 

 great moment, because it proves that there is no such thing as a drop of 

 normal size. At no degree of slowness of dropping do drops assume a size 

 unaffected by even a slight change in the rate of their sequence. Hence, 

 whenever a comparison has to be made between the sizes of different drops, 

 we shall have to eliminate this source of difference by taking dj'ops which 

 follow at exactly the same rate. 



About the rate at which the diminution of size takes place for equal 

 increments of fft, the Table gives us httle information beyond the fact that. 



