770 THE POPULAR SCIENCE MONTHLY. 



them in fact as particles of degraded bioplasm. This theory is not in 

 favor, or rather is not fashionable, at the present time. One objection 

 to its validity is constituted by the fact that particles of living animal 

 matter die very rapidly after they have escaped from the body, whereas 

 many contagious germs preserve their vitality and capacity for evil 

 for a very long time. 



Another theory which was promulgated some twenty years ago was 

 to the effect that the contagious particles are of the nature of those 

 low vegetable organisms which are termed fungi. This view gains 

 support from the manner in which these bodies increase in number 

 when planted in a suitable soil, and the power which they possess of 

 decomposing many organic substances. The fact, already referred to, 

 that several diseases of the skin and hair in men and animals are un- 

 doubtedly due to fungi, also tends to recommend this theory. Recent 

 experiments, however, have shown that these organisms, capable as 

 some of them are of growth and development on the surface of the 

 body, do not possess the power of growth and reproduction within the 

 body, and it is therefore unlikely that they should be the causes of dis- 

 ease in which the system is charged with poisonous materials. 



A third theory is one which is exti'emely popular at the present day, 

 advocated as it is by investigators of the highest repute. It is almost 

 needless to say that I refer to the view which credits certain minute 

 organisms, termed bacteria, with the power of causing the infectious 

 diseases — that is, with being in themselves the poisonous agents. So 

 firm is the hold that this view has obtained that " disease-germs " and 

 " bacteria " are used as though they were synonymous terms. It is, 

 moreover, probable that more experiments have been made with refer- 

 ence to bacteria than on any other subject whatever. 



The term " bacterium " signifies a rod, and many of these organisms 

 are minute, rod-shaped bodies. They or their germs are very widely 

 diffused throughout nature ; they swarm in the air and in water, espe- 

 cially if containing organic matter, and are likewise found in great 

 numbers within the bodies of men and animals. Any one w r ho pos- 

 sesses a microscope with a magnifying power of five hundred diame- 

 ters can readily examine a very common form of bacterium. It is only 

 necessary to take a glass of ordinary water from a spring or river, and 

 to leave it in a room exposed for some days to the air. A thin coat- 

 ing, looking like a deposit of fine dust, is formed on the surface of the 

 water ; this dust consists of myriads of bacteria, which are readily seen 

 when a drop of the water is examined. The bacteria are found to be 

 in several stages of transformation : some are in long, jointed rods, 

 others represent one or more detached portions of these rods, and others 

 appear as extremely minute, rounded particles. The rods are capable 

 of movement, and they are seen to wriggle through the fluid like small 

 eels or snakes. The minute, rounded particles are the spores, which 

 eventually become rod-shaped bodies. 



