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For instance, Geissler's pump requires a considerable number of taps, that of Alvergniat and Toepler is very fragile in consequence of its complicated system of tubes connected together, and that of Sprengel is only suitable for certain purposes. The new mercury pump constructed by Messrs.
So far back as 1793, the wonderful work of Sprengel had established, beyond any reasonable doubt, the fact that, in a large number of cases, a flower is a piece of mechanism the object of which is to convert insect visitors into agents of fertilisation.
Originally the two bulbs were joined by a tube which was connected to a Sprengel pump. When a high vacuum had been reached, first the connecting tube, and then the bulbs, were sealed off; they are therefore of the same degree of exhaustion.
If, however, adaptations of this kind were to be explained by natural selection, it was necessary to show that the plants which were provided with mechanisms for ensuring the aid of insects as fertilisers, were by so much the better fitted to compete with their rivals. This Sprengel had not done.
His hope with regard to his fame from these works was fulfilled, for they were printed as late as 1515 at Leyden, and Sprengel declared them the best compendium of simple remedies and diet that we have from the Arabian times. One of his translators into Latin has called him the monarch of physicians. Some of his maxims are extremely interesting in the light of modern notions on the same subjects.
The effectiveness of the metal tube as an electrostatic screen depends largely on the degree of exhaustion. At excessively high degrees of exhaustion which are reached by using great care and special means in connection with the Sprengel pump when the matter in the globe is in the ultra-radiant state, it acts most perfectly.
Alas for the irony of fate! Under Darwin's interpretation the very "defects" which had rendered Sprengel's work a failure now became the absolute witness of a deeper truth which Sprengel had failed to discern. One more short step and he had reached the goal. But this last step was reserved for the later seer.
In very many other cases, though there be no special mechanical contrivance to prevent the stigma of a flower receiving its own pollen, yet, as C. C. Sprengel has shown, and as I can confirm, either the anthers burst before the stigma is ready for fertilisation, or the stigma is ready before the pollen of that flower is ready, so that these plants have in fact separated sexes, and must habitually be crossed.
Platinum is chosen because it expands and contracts with temperature about the same as glass, and hence there is little chance of the glass cracking through unequal stress. The vacuum in the bulb is made by a mercurial air pump of the Sprengel sort, and the pressure of air in it is only about one-millionth of an atmosphere.
Even Freind and Sprengel, admirable historians, though more thoughtful and judicious in their criticisms, seem for the moment to have forgotten or overlooked the true character of the Compendium. Now, what precisely is Gilbert's Compendium designed to be? In the words of its author it is
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