270 BULLETIN OF THE BUSSEY INSTITUTION. 
It was noticed in one or two instances that old putty which had 
been left in the air long enough to become rather dry and hard 
was less readily eaten by the mice than fresh putty that had just 
been prepared. Doubtless there is a point when putty in drying 
becomes so hard and firm, —7.e. when the oil it contained has 
undergone such complete chemical change, —that mice will no 
longer eat any of it. On this supposition it is easy to explain the 
fact, well known to builders, that mice are particularly liable to 
infest new buildings, at least in cities, for so long as the fresh 
putty of the woodwork continues to be edible the animals will have 
comparatively little difficulty in procurin® a supply of food. There 
can hardly be any doubt that in such situations, in the lack of 
other eatables, the mice must often be compelled to supplement 
the oil ration with such animal food as they may be able to find 
in the shape of insects, larvee, and worms. It is not unlikely in- 
deed that, by destroying such pests, the mice may occasionally do 
some small good to the householder. The case is perhaps akin 
to that of the cosmopolitan house-sparrow — in some sort a flying 
mouse — who amid numberless acts of harm and annoyance, does 
occasionally perform one which suggests some slight justification 
for his existence, — to speak in the economic, utilitarian sense. 
After the mice of my experiments had become accustomed to 
eat large quantities of ordinary putty, as above described, several 
other mixtures of pigments and oil were offered them, as will 
appear from the following statements : — 
Red Ochre. A quantity of pure red oxide of iron (rouge) in a 
very finely divided state, was made into balls with raw linseed oil, 
and 11.1 grms. of the mixture * were placed in the cage. In the 
course of the first twenty-four hours the three mice ate 1.3 grms. 
of it. Their dung meanwhile became red instead of white, and 
much less bulky than it had been when putty was eaten. But on 
repeating the same dose of the red oxide on the following day, the 
mice would not eat any of it. The balls of red ochre being then 
removed and 12 balls of ordinary whiting-putty put in their place, 
all of them were eaten in a short time. Five hours after this sub- 
stitution of whiting for red ochre it was noticed that an abundance 
of the large white dung had already collected in the cage. 
It is not easy to explain the refusal of the mice to eat the mix- 
ture of red ochre and oil, since the insoluble ochre can hardly have 
* Containing 8.3 grms. of the oxide and 2.8 grms. of oil. 
