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THE POPULAR SCIENCE MONTHLY.

colonized from that source, and that the development of great and cultivated nationalities was the result of ages of quiet residence in countries which favored by their climate and resources the special phase of development which we here find recorded.

As to the date of the planting of the first seeds of this civilization we can only say that it is lost in the obscurity of the past. Everything indicates that some of the monuments in the category we have reviewed are among the oldest records of the human race; and it is certain that the gradual growth and spread of this civilization, the long noonday of its maturity, and its progressive decadence—which began long before the advent of the Europeans—must be measured by thousands of years. Thus it will be seen that in antiquity this indigenous and peculiar American civilization takes rank with that of the Egyptians, Assyrians, Hindoos, and Chinese, and in respect to culture, numerical importance, and territorial area will bear comparison with either.

 

WHAT ARE DIATOMS?
By EMILY L. GREGORY,

OF BARNARD COLLEGE.

SINCE the microscope has become so familiar in our homes and ordinary places of resort, many terms are frequently heard which have an unfamiliar sound. For example, a lady asked the other day, with a laugh over the open confession of ignorance: "What are diatoms? I hear the word used very frequently, and with such an air of acquaintanceship and familiarity, that one must suppose they are the most common, every-day affairs, and yet I must confess I have never seen one and don't know really what they are."

Thinking possibly there might be others interested in a brief description of this curious plant, the following story is told of a visit paid this summer to a gentleman said to know all about diatoms. The plants in question are so small as to be seen only with the aid of the microscope; those of ordinary size, when magnified about three hundred and fifty diameters, appear about a quarter of an inch long. Others are much larger. They are curious little plants with a silica shell, which, in certain places, is provided with little apertures through which living parts of the plant protrude. In this way they are enabled to move about freely in the water by which they are generally surrounded, for, though they are not all strictly water plants, they all need considerable water to enable them to thrive, and so are always found in wet places.