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due in all cases to superior health or robustness of constitution. Instances are familiar of one or two escaping, when an infectious fever runs through a family, and those who escape the infection are by no means always the strongest. The baby often escapes while older children are attacked. Persons above 40 years of age are less liable to typhoid fever than those who are younger; yet not from greater strength of constitution; for, if attacked, they are more likely to succumb.
Is not the immunity in such cases due to some protecting cause, which we do not discern? And, if it be, is the cause one, which we could put into operation, as we use vaccination for the prevention of small-pox? Do not some of these familiar facts seem as if they were pregnant with secrets, which lie before us, as Jenner's lay before him, ready to be disclosed to clear, impartial eyes(s)?
The three signal examples I have given illustrate three characteristic ways in which Physicians "search out the secrets of Nature;" and invention is ever busy in helping their search by devising new instruments and new methods of investiga-
