12 



FARMERS BULLETIN 798. 



ably taken in largely by ingestion and to a less extent b}^ absorption, 

 and kill through cumulative action of the poison. The nicotin dips 

 are probabh^ taken in mainly bj'- absorption and to a less extent by 

 ingestion and respiration. The coal-tar-creosote and cresylic-acid 

 dips give off gases and are taken in mainly by respiration and pos- 

 sibly to a less extent by absorption. It is seemingly not a mere 

 question of bringing the parasite into temporary contact with the 

 dip, as the period of time -during which the poison exerts its action 

 is an important factor. Very few of the known dips will kill the 

 parasite immediately ; therefore the length of time the sheep are held 

 in the vat is not the sole determining factor, provided they are held 

 in the dip a sufficient length of time to saturate the fleece. The 

 length of the wool, the quantity of dip retained in the fleece, the 

 length of time that the dip remains active in the wool, and the nature 



Fig. 8. — Close view of portion of nock of lamb shown in figure 7, showing ticks 

 and puptc in wool. 



