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The variations in the tree, in M. Becquerel's own observations, appear as considerably less than a fourth of those in the atmosphere, and he has calculated, from observations made at Geneva between 1796 and 1798, that the variations in the tree were less than a fifth of those in the air; but the tree in this case, besides being of a different species, was seven or eight inches thicker than the one experimented on by himself.
A similar difference of progression is visible in the means, which differ most in spring and autumn, and tend to equalise themselves in winter and in summer. But it appears most strikingly in the case of variations somewhat longer in period than the daily ranges. The following temperatures occurred during M. Becquerel's observations in the Jardin des Plantes: Date.
There were many tedious and difficult steps to take before even our present knowledge, incomplete as it is, could be reached. However, Becquerel's fortunate accident of the plate developing was the beginning of the long series of experiments which led to the discovery of radium which already has revolutionized some of the most fundamental conceptions of physics and chemistry.
In 1895 came the discovery of the X-rays by Röntgen in Germany, to be followed in a year by Becquerel's discovery of spontaneous radio-activity, and in a couple of years by the remarkable further discovery, made by Madame Curie, of what was termed "radium," a substance that went on producing heat de novo, keeping itself permanently at a higher temperature than its surroundings, and spontaneously producing electricity.
But this is precisely the same conclusion as we have already arrived at with regard to individual trees; and in Herr Rivoli's table, what we see is just another case of what we saw in M. Becquerel's the different progression of temperatures.
The results are perfectly congruous. Becquerel's observations were made under wood, and about a hundred yards outside in open ground, at three stations in the district of Montargis, Loiret. There was a difference of more than one degree Fahrenheit between the mean annual temperatures in favour of the open ground.
Becquerel's views have been amply confirmed by recent extensive experiments on the bleak, stony, and desolate plain of the Cran in the Department of the Bouches-du-Rhone, which had remained a naked waste from the earliest ages of history.
According to Becquerel's measurements, the quantity falling on bare and on wooded soil respectively was as 1 to 0.07; 1 to 0.5; and 1 to 0.6, or, in other words, he found that only from five-tenths to sixty-seven hundredths of the precipitation reached the ground. Comptes Rendus de l'Academie des Sciences, 1866.
The radio-activity of salts of uranium was proved not to be increased or diminished when these salts had been shielded for five years from the action of light by keeping them in leaden boxes. Shortly after Becquerel's discovery, experiments proved that salts of the rare metal thorium are radio-active.
It will be observed that I have omitted up to the present point all mention of station C. I do so because M. Becquerel's language leaves it doubtful whether the observations made at this station are logically comparable with those made at the other two.
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