invisible bacteria which it contains may be destroyed, even as many as ninety per cent. But still large numbers may remain alive, for many species are quite invulnerable to the action of cold. It has been found that in ice formed from water containing many bacteria, such as water with sewage contamination, the snow-ice almost invariably contains many more living bacteria than the more solid, transparent part; so that the snow layer should be especially avoided in ice obtained from questionable sources. Unfortunately, the bacteria which cause typhoid fever are not readily killed by cold, and may remain alive for months, fast frozen in a block of ice.
As the neighborhood of our ice-fields becomes more thickly-settled, and the demand for ice also increases, the danger that frozen filth will be served out to consumers of ice will increase likewise. It is fortunate that the artificial process stands ready to shield us from this peril.
Utility has not entirely monopolized the artificial production of ice; it has been made to serve sport as well. About 1875 a Mr. Gamgee, in England, constructed a rink of artificial ice for summer skating, and several others have been made in that country. In 1889 an immense rink of this kind was established in Paris, circular in form and one hundred and seventy feet in diameter. Around the sheet of ice was a promenade over seven yards wide,
Fig. 4. — Arrangement of the Expansion Coils in a Rink in Paris.
and outside of this were placed seats for spectators, a band-stand, etc., the whole being covered by an arched roof. The arrangement of this rink is shown in Fig. 3. The ice-sheet was formed on a concrete bed, upon which lay an immense coil of iron pipe, as shown in Fig. 4, having a total length of ten miles. The pipe