VII. _Illuminating Gas_ not only withdraws from the air of our rooms a considerable amount of oxygen, but fills them with noxious products of combustion. All this may be avoided in the future by the introduction of the electric light. VIII. _Sanitary Surveys_.--Dr. Bowditch has shown that a thousand deaths from consumption in Massachusetts are due to a wet and retentive soil, and this fact alone will show the importance of sanitary surveys of the country, such as that made of Staten Island by Professors Newberry and Trowbridge, who determined the influence of the surface soil, of the underlying rock, its porosity, its bedding and its joints, upon the drainage and upon the local climate and health. A similar survey of Hudson county, New Jersey, has been recently made by L. B. Heard, C.E. IX. _Composition of the Atmosphere_.--The English government has been obliged to appoint the celebrated Dr. Angus Smith to examine the effects of atmospheric contamination. In Philadelphia there is scarcely a house front that is not disfigured by the stain of magnesia and lime salts, caused by acid vapors in the atmosphere. A discussion followed, which was introduced by Mr. Collingwood, who remarked that the problem of the sewage of cities was still far from being solved. Though the recent experiments in England on utilizing sewage for agricultural purposes by filtration and otherwise were reported to be successful, we had only dodged the question in this country. Our sewage is still emptied into rivers to poison the water of cities further down their course. When the country becomes more thickly settled, this will answer no longer. It was also stated that while gas in large chandeliers could be made an effective means of ventilation, there was another objection to its use in the fact that the soil of the city was everywhere impregnated with it from leaky mains, thus causing poisonous exhalations and an insufferable odor whenever the ground was opened. Attention was also called to the evil effects of the system of tenement houses, which led to an unfavorable comparison of the health and morality of New York with those of cities like Philadelphia and Cleveland, that abound in small homes. Dr. Minor attributed disease to what Richardson calls "ultra-microscopic molecular aggregates," which always exist in the air, but take hold of us only when our vitality is reduced to a certain point. It has been shown that decay is absolutely impossible in vessels from which they are excluded. But for them the earth would now be heaped with the undecomposed remains of animals and vegetables. According to this view, the future efforts of sanitary science must be simply in the direction of learning how to protect ourselves against the "ultra-microscopic molecular aggregates." C. F. K. * * * * * FELLING TREES BY ELECTRICITY.