312 GERMICIDAL ACTIVITY OF EUCALYPTUS OILS, ii., 



matter of the environment. It is, therefore, evident that a com- 

 mon medium must be employed when a comparison is desired, 

 and the Rideal- Walker test is that which is generally used. In 

 it, the activity of the disinfectant is not interfered with, and 

 although the test has its weaknesses, it has been very useful in 

 giving us some idea of the value of disinfectants in the absence 

 of organic matter. The test is easy to do, and it should give 

 approximately similar results when made by different workers. 



The Eucalyptus oils are partly soluble in water and partly 

 emulsified, much depending upon the oil and upon the quantity 

 of water. Cineol or Eucalyptol, the chief constituent of many 

 of the oils, is soluble in about 300 parts of water, while the other 

 constituents are so insoluble that many give opalescent dilutions 

 with 2,000 parts of water. Their power of forming emulsions is 

 probably of value, for, as I think Martin has pointed out, the 

 adsorption by bacteria from emulsions is greater* than from 

 solutions, and the adsorption is the first step in the destruction 

 of bacteria by disinfectants. 



In preparing the dilutions of the oils, two methods were 

 employed — the mass and the droplet methods. In the first, a 

 certain weight of oil was shaken with a calculated quantity of 

 water, usually 199 times the weight of the oil, and from this 

 strong emulsion the weaker dilutions were made by adding the 

 requisite quantities of water. In working with the oils, one is 

 struck with the tenacity with which they adhere to the glass of 

 the flasks and pipettes, and with the idea of minimising any 

 irregularity rising from this phenomenon, the droplet method 

 was used as an alternative. One point in favour of the droplet 

 method is that, when only small quantities of the oil are avail- 

 able, there is very little waste. A capillary pipette was made 

 and kept for dropping. Each oil under examination was tested 

 daily or weekly, according to the laboratory temperature, by 



* "Adsorption undoubtedly plays a large part in many forms of disin- 

 fection, and confers upon emulsions, as contrasted with solutions, con- 

 siderable advantages." Somerville, Cantor Lectures, 1913. Roy. Soc. of 

 Arts. 



