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CORRESPONDENCE. 
%* The Editor does not hold himself responsible for statements of fa&s or 
opinions expressed in Correspondence, or in Articles bearing the signature 
of their respe&ive authors. 
THE IMMUNITY CONFERRED BY AN ATTACK 
OF CERTAIN CONTAGIOUS DISEASES. 
To the Editor of the Journal of Science. 
Sir, — Allow me to call your attention to a coincidence. In the 
“ Mason College Magazine ” for October, published at the begin- 
ning of that month, a correspondent who signs himself “X. Y. Z.” 
writes as follows : — “ It seems to be, though I am probably ut- 
tering a mere truism, long recognised by all acquainted with the 
subject, that the germ theory gives a clue to the puzzle. The 
principle seems to be that of the rotation of crops. When the 
smallpox or vaccine germs are planted in the human body, they, 
as it were, exhaust the soil by withdrawing from it the special 
nutriment which they and their kind require. Until the element 
withdrawn is renewed, which may happen sooner in some con- 
stitutions than in others, the patient is not liable to a return of 
the disease.” 
In the “ Pall Mall Gazette ” for Ocftober 30th Professor John 
Tyndall says : — “ He (Sir Thomas Watson) once asked me how 
I supposed the immunity just referred to was to be accounted 
for ? I acknowledged the difficulty, but stated at the same time 
that the germ theory came nearest to a satisfactory solution. A 
certain amount of mineral matter was known to be necessary to 
the constitution of a tree. Without it the tree could not grow, 
although in comparison with the other constituents of the wood 
its quantity might be infinitesimal. The exhaustion of a soil 
often meant the removal from it of a minute but necessary con- 
stituent. Reasoning from analogy I ventured to express the 
opinion that the rarity of second attacks of communicable disease 
was due to the removal from the system, by the first .parasitic 
crop, of some ingredient necessary to the growth and propagation 
of the parasite.” 
Here, surely, is a most curious coincidence. — I am, &c., 
O. P. Q. 
