14 Transactions of the Society. 



passed over unless a very thin stratum of the fluid which holds 

 them in suspension be examined. Though so small, they are 

 probably bodies of the very highest importance in the disintegra- 

 tion of sewage compounds. The germs of these organisms are 

 excessively minute, many being less than the 1/100,000 of an in. 

 in diameter, whilst the smallest are probably not to be seen when 

 amplified by the highest magnifying powers at our disposal. So 

 very small are they that they must grow for some time before they 

 are of a size sufficient to be rendered visible by an objective which 

 magnifies upwards of 5000 diameters. Bacteria germs exist 

 everywhere in countless multitudes, not only in air and in water 

 and on the surface of every kind of matter, but in the interior of ^ 

 bodies living as well as non-living wherever fissures exist, and 

 chinks are seldom absent through which germs so minute can pass. 

 Not only are bacteria always to be found upon every part of the 

 surface of all living beings, but they exist within the blood, and in 

 the very substance of the tissues however distant from the external 

 surface of the body, and however far from any direct communica- 

 tion with the outside air. In all animals and in all plants, at all 

 temperatures consistent with life — in every part of the world — 

 bacteria are living at this moment, and they have lived, and pro- 

 bably in the same way as they live now, in every period of the 

 world's history from the earliest dawn of life. Soon after the 

 death, and in many instances long before the death of a man or an 

 animal has taken place, the bacteria germs, which have been 

 dormant in the tissues and fluids, begin to grow and multiply 

 enormously, so that in a very short time every part is freely per- 

 vaded with countless hosts which soon stop all ordinary action and 

 efiace all characteristic structure. Then begins that long series of 

 changes which ends at last in the formation of products of com- 

 paratively simple character and very stable nature. 



Extremely minute division of the organic matter of sewage 

 and its equable diffusion through a large volume of water in 

 constant motion are favourable to the conversion by bacteria, of 

 noxious matter into chemical compounds, which are inodorous and 

 harmless, and which undergo but slight change whether moist or 

 dry, and which are usually at last disposed of by becoming the 

 food of plants. In the case of sewage this desirable change into 

 innocuous compounds is rendered very slow in consequence of the 

 matter not being spread out in a sufficiently thin layer to be 

 quickly appropriated by the bacteria. 



The rate of growth and multiplication of bacteria varies greatly 

 at different periods of the year. These organisms are not 

 destroyed by ordinary cold ; nay, there is evidence that bacteria 

 multiply after having been exposed even to intense cold, but of 

 course very slowly as compared with their rate of increase under 



