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FIFTH REFORT— 1835 . 
greatly the numbers of observations, that such sources of fallacy, 
attending any single cases, can be avoided; and general laws, 
touching the influence of such causes, be satisfactorily deduced. 
Thus it is in general only by observing that a particular dis¬ 
ease affects a much greater number of those persons who are 
known to have been exposed to the agency of a particular ex¬ 
ternal circumstance, than of those who are not known to have 
been exposed, that we learn that such circumstance has power to 
cause that disease. It is very seldom, particularly in civil life, 
that we can have observations, as to the influence of such a 
cause, of the nature of the experimentum crucis ; i. e. when 
all other circumstances in the condition of the persons observed 
are exactly alike, excepting only the presence of that cause in 
one set of cases, and its absence in another. But it may always 
be presumed that out of a very great number of cases in which 
one condition has been uniformly present, all other conditions 
must have been applied very variously ; and therefore, by very 
greatly multiplying the number of observations, where one 
alleged cause has been applied, we may ultimately get rid of 
the source of fallacy, resulting from the varying conditions of 
each single observation, and from a fair estimate of the efficacy 
of the particular cause in question. 
Tims also the experience of an individual, even if carefully 
preserved, goes but little way in ascertaining the effects of sea¬ 
sons, of localities, of occupations, or modes of life on the mor¬ 
tality of any given disease, because in every individual case 
which has been under his observation, the influence of any one 
of these causes must always have been combined with that of 
others, which may have determined the. result; but if the expe¬ 
rience of a very great member of individuals on the mortality 
from that disease, under the influence of one of these causes, is 
exhibited at once, it may fairly be presumed that all acciden¬ 
tally concurrent causes must have acted so variously on so great 
a multitude, that the irregularities thence arising must have de¬ 
stroyed one another, and that the influence of permanent and 
general laws only will be perceptible in the result. Many at¬ 
tempts have accordingly been made by medical men to acquire 
more certain information, as to the comparative efficacy of dif¬ 
ferent causes of disease and mortality, than the experience of 
individuals can supply, by reference to registers of deaths, kept 
in different situations, and extending to large numbers of per¬ 
sons and to long periods of time. But these attempts have 
been in a great measure frustrated, or at least their results ren¬ 
dered much less certain and important than they would other¬ 
wise have been in this country, by the imperfect and irregular 
