THE MEMBRANES. 95 



The first intimation of infection is, of course, the fact that the cells 

 will not develop maximum pressures. On removing the solutions, they 

 are found always to have a more or less greenish color, and the absence 

 of this color is a sure sign that the membrane has not been attacked 

 by the fungus. If the destruction of the membrane by the penicillium is 

 far advanced, there are also found, upon the bottom of the cell, minute 

 grains of a substance whose color varies from green to blue. When the 

 fungus is allowed to grow in a solution of sugar to which some membrane 

 material (copper ferrocyanide) has been added, the solution becomes 

 green, then blue, and finally brown from suspended iron hydroxide, as 

 if the whole of the nitrogen of the ferrocyanide had been appropriated. 



The restoration to a usable condition of a membrane which has been 

 attacked by this penicillium is a work of some months. The quickest 

 method of killing the fungus is to place the wet cell under a bell jar and to 

 develop in the inclosed space gaseous hydrocyanic acid by dripping dilute 

 hydrochloric acid into a dish containing potassium cyanide. Though it 

 grows vigorously in dilute solutions of copper sulphate which are exposed 

 to the air, it soon dies in a saturated solution of thymol. It is quickly 

 killed by formaldehyde in dilute solution. Phenol and salicylic acid 

 appear to be less poisonous to it than thymol. 



After destroying the penicillium, it is necessary to begin anew and 

 to repeat in every detail the series of operations employed in building 

 up and seasoning membranes. 



Because of the laborious character of the measures which must be 

 resorted to for the restoration of the membranes, every possible pre- 

 caution is taken to prevent infection. Some of these preventives have 

 already been mentioned incidentally — for example, the boiling of all 

 water which comes in contact with the cells, the careful covering of all 

 vessels, the soaking of the cells in thymol water, and the addition of 

 minute quantities of thymol to the liquids within and without the cell 

 when a measurement of pressure is to be made. Another precaution 

 consists in the occasional disinfection of the baths at high temperature 

 (from 70° to 80°) with the vapors of formaldehyde, which are circu- 

 lated within the inclosed spaces by means of fans. The interior walls 

 of the baths, together with all their fittings, such as wires, etc., are 

 frequently washed with a solution of formaldehyde. In the older form 

 of "rectangular" bath the water and air spaces were not carefully 

 separated. The upper or air space was therefore always saturated 

 for the given temperature with water vapor, producing a condition 

 which was especially favorable to the growth of the penicillium. It 

 was found impossible to rid these baths of infection for any length of 

 time. They were, therefore, all reconstructed with vapor-tight par- 

 titions between the two compartments. It was afterwards practicable 

 to keep the air spaces so dry (by means of desiccating agents) that the 

 fungus could not grow. 



