ARTICLES 229 



thus some doubt as to the exact value of the constant to be used, 

 but this difficulty is overcome by Morgan by using the falling drop 

 method to give values of the surface tension relative to that of 

 benzene as determined by the capillary rise method. As a 

 general method, Morgan's has the advantage over the capillary 

 rise method that the surface of the drop is a large one, and, 

 moreover, is constantly renewed, so that the effect of small 

 quantities of impurities is much reduced. Morgan's apparatus 

 can be used for all temperatures up to 90" C, and errors due to 

 evaporation are avoided. He has established, by a very careful 

 series of experiments, that for a given dropping tip the ratio of 

 the surface tension to the weight of the falling drop at the same 

 temperature is equal to a constant, i.e., 



^CeUe.t ^HaO.r ^i.f 



= C. 



wherefore Tj^f = Cm^i l* for any given tip. Each tip has to 

 be standardised by means of benzene or water using the values 



^CeHs.i^ 30-514 - 0-1321 / + 0-000082 t' 

 ^H^O.i = 75-872 - 0-154 t -{- 0-00022 t' 



The equation for benzene is derived from the average of all the 

 results of all accurate workers using the capillary rise method : 

 that for water was obtained by comparison with benzene using 

 the drop method. Another equation which Morgan finds to 

 hold rigidly is 



^ _ (288-5- j?-6) 



^CeHe./ -2-II5 (78M)^ 



where the density of the liquid is given by dt = 0-9002 — 

 0-001066 i. In using the method, there is one condition which 

 is essential, i.e. the drop must have a normal outline ; this is a 

 bag-like form with the sides parallel for a part of their length. 

 If the tip is too small, the drop bulges at the bottom, so that the 

 maximum diameter of the drop is greater than that of the tip, 

 the results being too high. If the tip is too large, there is a loss 

 of control over the formation of the drop, and the latter converges 

 at the bottom and breaks away at a different point to the 

 " normal " drop, and again gives too high results. It is found 

 that the conditions are satisfactory with tips having diameters 

 from 4-5 mm., giving drops with volumes greater than 0-0196 c.c. 

 to 5-5 mm., giving drops with volumes greater than 0-0239 c.c. 

 for all but one out of two hundred liquids studied ; the 

 exception is carbon tetrachloride, having T = 24-9 and d = 

 J -576, so that with a 4-5 mm. tip the volume of the drop is 



