TEXT-BOOK OF BACTERIOLOGY. 95 



In making these dilutions, there is one special method which 

 practice has shown to be the best and which usually proves suc- 

 cessful : from the first tube the " original " remove with the plati- 

 num loop three successive drops of the inoculated gelatin and 

 place them into a second tube the "first dilution;" out of this 

 again, three drops into a third tube the " second dilution." In the 

 one or the other of these three tubes the germs will generally be 

 found so distributed that the contents can be taken as the starting- 

 point for further researches. 



It is not necessary to follow these directions literally under all 

 conditions. On the contrary, the number of dilutions and also the 

 number of drops apportioned to each will have to be varied ac- 

 cording to circumstances. 



For the transmission of material by means of the platinum 

 loop, there are certain special manipulations which have proved 

 practically useful and are universally employed. 



With the left hand opened and the palm directed upward, 

 place the "original" tube between the thumb and fore-finger. 

 The mouth of the tube is turned toward the operator. It should 

 always be held, as much as possible, in a horizontal position, in 

 order that when the cotton-wool plug is removed no germs may 

 fall in from the air. Now place beside this first glass a second, 

 which is to be used for the first dilution; here, too, the opening is 

 directed toward the operator, and is about in the centre of the palm 

 of the hand. Remove the cotton-wool plugs by carefully twisting 

 them, first from tube II., then from tube I. ; the former is placed 

 between index and middle finger; the latter between the ring and 

 little fingers of the left hand. By always observing these direc- 

 tions we may avoid confounding the plugs. Next put the steril- 

 ized but cooled platinum loop into the first tube, take out a drop of 

 gelatin and transfer it to tube II., and by moving the wire up and 

 down endeavor to distribute the contents of the drop throughout 

 the liquid gelatin. Repeat this process a second and a third time 

 (it is unnecessary to heat the platinum loop anew each time). 

 When this is done the cotton plug is placed in tube I., which is set 

 aside, preferably into an empty tumbler. Then, just in the same 

 way the palm of the hand remaining always directed upward- 

 transfer from the first dilution to the second. 



It is desirable to always know which tube is the original, which 

 the first and which the second dilution. Therefore we mark them 

 from the beginning, either with labels or with a Faber wax-pencil, 

 or by twisting the cotton plug of the first dilution into one, that 

 of the second dilution into two long ends. 



