1889. ] of Struvite by Micro-organisms. 361 
of crystals seems to be increased by cultivation; it at first pro- 
duced a green colour rapidly and crystals slowly, now the order 
is reversed. These observations may indicate that the micro- 
organisms first of all secrete a ferment which decomposes the 
nutrient substance, so that they are therefore only indirectly 
concerned in producing the ammonia which is necessary for the 
formation of the crystals; but on the other hand certain micro- 
organisms which Dr Cunningham has recently obtained for me 
from the human mouth behave, when cultivated on agar-agar, so 
as to lead me to conclude their action is direct. They have pro- 
duced crystals on the second day after inoculation, and these 
crystals have been formed at the points of most active growth, 
that is immediately beneath the strongest colonies. In many of 
my older cultivations they are also formed in this position, while 
in others they are irregularly distributed through the agar-agar. 
Whether the crystals are produced directly by the micro-organisms, 
or indirectly by a ferment which is the product of their lives, it 
is a fact that they do appear sooner or later in nearly every one 
of the numerous tubes in which I have a micro-organism growing, 
and that they never are formed in any tubes free from contami- 
nation, and I have some of the latter in my possession which I 
have had under observation for more than fifteen months. I 
have also found crystals in tubes of agar-agar and nutrient gelatin 
where the only growth was a mould, and in others where it was a 
yeast. I have analysed the crystals and have found them to 
consist of the double ammonium-magnesium phosphate ; further, 
Mr Solly, of the mineralogical department, has kindly examined 
them for me, and has found their crystalline form identical with 
that of the mineral Struvite. My theory is that the micro- 
organisms produce the ammonia from the nitrogenous organic 
matter in which they are growing, and that it then combines with 
the magnesium phosphate present both in the nutrient gelatin 
and in the agar-agar to form the double salt. This formation of 
ammonium- magnesium phosphate in the tubes by micro-organisms 
appears to explain the formation of the mineral Struvite in nature. 
The latter has been found in old graveyards, under the floors of 
stables, and in guano, in exactly the places where organic matter 
is undergoing decomposition in the presence of magnesia and 
phosphoric acid. Moreover it is probable that the decomposition 
of organic matter only takes place through the intervention of 
low forms of life, for it is well known that when substances so 
very prone to undergo decomposition as meat, jelly, milk, urine, &c. 
have been sterilised by heating, they may be kept exposed to the 
atmosphere for almost indefinite periods without putrification 
taking place if the vessels containing them are plugged by cotton 
wool so that the air is filtered, and so is freed from micro-organisms 
