nected with putrid decomposition were regarded as a sufficient cause for the development of fever, small-pox, etc. It can not be denied that gaseous matters, notably sulphureted hydrogen, may act as poisons and cause many serious symptoms, but it has never been shown that infectious diseases originate in this manner. It is contrary to all that chemistry teaches us, that sulphureted hydrogen or ammoniacal vapors inhaled by the lungs should increase within the body and cause it to become a center of infection; and we know likewise that ordinary poisons—e. g., arsenic or morphia—fatal as their effects may be to one individual, have no power of increase and propagation after being once taken. It is therefore evident that the poisons of infectious diseases must be something of an entirely different nature. We know that they multiply in the system to an almost infinite extent, and that every one of the myriads of atoms thus developed is as potent for evil as the atom from which it originated. The possession of this and other properties clearly indicates that the contagious agencies are independent living organisms, capable of growth and reproduction. It has long been known that certain diseases of the skin—e. g., ringworm—are caused by the presence of parasites, which very rapidly increase, and can be easily recognized under the microscope.
In the case of some three or four of the infectious diseases it would seem that the poison has really been discovered. On examining vaccine matter, the contents of the pocks in small-pox, and discharges in glanders, the microscope shows a vast number of infinitely minute particles, which appear as glistening points. Some of these are even less than the fifty-thousandth of an inch in diameter, and it therefore follows that very high powers are necessary for their detection. Such particles, obtained from vaccine lymph, have been washed in water; the water when inoculated did not produce any effect, but the washed particles were found to have retained their potency. It seems fair to infer that the contagious agents of the other infective diseases would resemble in their physical characters that of vaccine, and the nature of such particles is the important problem that offers itself for solution. They are supposed by some, and notably by Dr. Beale, to be of an animal origin, and to consist of elementary living matter, termed bioplasm. Such particles may be easily transferred from an infected to an uninfected organism, in which they will manifest their own specific powers, and grow and multiply almost indefinitely, exciting in their new home a series of changes resembling those which characterized their presence in the one from which they were derived. This account is certainly correct as regards the virus of vaccine, but it does not precisely define the nature of the particles or tell us anything of their origin. Dr. Beale, however, states that particles of contagious bioplasm are not generated in the organism of the infected animal, but are introduced from without, and were originally derived by direct descent from the bioplasm of the body of man or animal. He regards
