84 CHEMOTAXIS AND CHEMICAL DISINFECTION 



tive media. Before this is done, however, the threads or garnets, as the 

 case may be, must be freed from any adherent disinfectant. Washing with 

 distilled water is not enough for this, they must be rinsed with ammonium 

 sulphide, and the soluble metallic salts precipitated and rendered harmless. 

 Only in this way is it possible to obtain reliable results, for the small 

 quantity of disinfectant adherent to the spores, although not sufficient to 

 kill them under ordinary circumstances, might do so during the stage of 

 imbibition, and would certainly destroy the germinating rod, thus giving 

 a false idea of the disinfecting power of the substance. In the examples 

 given in Table III, from 15,000 to 20,000 spores were killed in the stated 

 times. The concentration is given in per cents., and also in terms of 

 'molecular dilution' ; for instance, 'Corrosive Sublimate, 16 litres,' means 

 that sixteen litres of the solution contain the molecular weight of HgCl 2 

 (i.e. 271) expressed in grams, so that 100 c.c. of the solution contains 

 flff grams = 1-7 grams. This method of calculation, so much used in 

 modern chemistry, is given because it permits an easy comparison of 

 different salts a comparison of molecule with molecule, so to speak. 



According to Koch absolute alcohol, concentrated glycerine, concentrated 

 NaCl solution, and distilled water do not destroy anthrax spores, even after 

 acting on them for months. The table shows that the halogen elements 

 (chlorine, bromine, iodine) are the most powerful, and, among the metallic 

 salts, corrosive sublimate. Nitrate of silver has some little effect, but sulphate 

 of copper and acetate of lead are powerless. Free acids or free alkalies must 

 be very strong to act as disinfectants. Chromate of potash, too, although it 

 is a powerful oxidizing agent, has little germicidal power, whilst potassic 

 permanganate, far less effective as an oxidizer, is a powerful disinfectant. 



Tables II and III show the great difference between spores and vegeta- 

 tive cells in their behaviour to disinfectants. Five per cent, carbolic acid, 

 for instance, which kills tubercle bacilli in thirty seconds, does not destroy 

 anthrax spores in twenty-four hours ; with o-i per cent, sublimate, the times 

 are ten minutes and sixty or eighty minutes respectively. Absolute alcohol, 

 powerless against anthrax spores, kills tubercle bacilli in five minutes. The 

 resistant power of the spores depends, no doubt, principally on the great im- 

 permeability of the spore wall to dissolved substances of all kinds. This 

 property is shared by the cysts and membranous coatings of all low organisms 

 in the resting-stage, and by the seeds of plants. A resting-stage of long 

 duration is indeed inconceivable without some such protection. In plant 

 seeds and algal spores the impermeability is caused by the deposition in 

 the membrane of fats and resinous bodies, and possibly the spore wall in 

 bacteria is protected in the same way. Another cause of the durability of 

 spores is the density of the protoplasm, which contains less water than it 

 does in the vegetative cell. 



The great efficacy of corrosive sublimate as a disinfectant seems at first 



