The explanation of these curves is discussed in detail in [chapter X], [section 212].
185. Physical and chemical properties of the active deposit. On account of the slow decay of the activity of the active deposit from the thorium emanation, its physical and chemical properties have been more closely examined than the corresponding deposit from radium. It has already been mentioned that the active deposit of thorium is soluble in some acids. The writer[[281]] found that the active matter was dissolved off the wire by strong or dilute solutions of sulphuric, hydrochloric and hydrofluoric acids, but was only slightly soluble in water or nitric acid. The active matter was left behind when the solvent was evaporated. The rate of decay of activity was unaltered by dissolving the active matter in sulphuric acid, and allowing it to decay in the solution. In the experiment, the active matter was dissolved off an active platinum wire; then equal portions of the solutions were taken at definite intervals, evaporated down in a platinum dish, and the activity of the residue tested by the electric method. The rate of decay was found to be exactly the same as if the active matter had been left on the wire. In another experiment, an active platinum wire was made the cathode in a copper sulphate solution, and a thin film of copper deposited on it. The rate of decay of the activity was unchanged by the process.
A detailed examination of the physical and chemical properties of the active deposit of thorium has been made by F. von Lerch[[282]] and some important and interesting results have been obtained. A solution of the active deposit was prepared by dissolving the metal which had been exposed for some time in the presence of the thorium emanation. In most cases the active matter was precipitated with the metal. For example, an active copper wire was dissolved in nitric acid and then precipitated by caustic potash. The precipitate was strongly active. An active magnesium wire, dissolved in hydrochloric acid and then precipitated as phosphate, also gave an active precipitate. The activity of the precipitates decayed at the normal rate, i.e. the activity fell to half value in about 11 hours.
Experiments were also made on the solubility of the active deposit in different substances. A platinum plate was made active and then placed in different solutions, and the decrease of the activity observed. In addition to the acids already mentioned, a large number of substances were found to dissolve the active deposit to some extent. The active matter was however not dissolved to an appreciable extent in ether or alcohol. Many substances became active if added to the active solution and then precipitated. For example, an active solution of hydrochloric acid was obtained by dissolving the deposit on an active platinum wire. Barium chloride was then added and precipitated as sulphate. The precipitate was strongly active, thus suggesting that the active matter was carried down by the barium.
186. Electrolysis of solutions. Dorn showed that, if solutions of radiferous barium chloride were electrolysed, both electrodes became temporarily active, but the anode to a greater degree than the cathode. F. von Lerch has made a detailed examination of the action of electrolysis on a solution of the active deposit of thorium. The matter was dissolved off an active platinum plate by hydrochloric acid, and then electrolysed between platinum electrodes. The cathode was very active, but there was no trace of activity on the anode. The cathode lost its activity at a rate much faster than the normal. With an amalgamated zinc cathode on the other hand, the rate of decay was normal. When an active solution of hydrochloric acid was electrolysed with an electromotive force smaller than that required to decompose water, the platinum became active. The activity decayed to half value in 4·75 hours while the normal fall is to half value in 11 hours. These results point to the conclusion that the active matter is complex and consists of two parts which have different rates of decay of activity, and can be separated by electrolysis.
Under special conditions it was found possible to make the anode active. This was the case if the anion attached itself to the anode. For example, if an active hydrochloric solution was electrolysed with a silver anode, the chloride of silver formed was strongly active and its activity decayed at a normal rate. The amount of activity obtained by placing different metals in active solutions for equal times varied greatly with the metal. For example, it was found that if a zinc plate and an amalgamated zinc plate, which show equal potential differences with regard to hydrochloric acid, were dipped for equal times in two solutions of equal activity, the zinc plate was seven times as active as the other. The activity was almost removed from the solution in a few minutes by dipping a zinc plate into it. Some metals became active when dipped into an active solution while others did not. Platinum, palladium, and silver remained inactive, while copper, tin, lead, nickel, iron, zinc, cadmium, magnesium, and aluminium became active. These results strongly confirm the view that excited activity is due to a deposit of active matter which has distinctive chemical behaviour.
G. B. Pegram[[283]] has made a detailed study of the active deposits obtained by electrolysis of pure and commercial thorium salts. The commercial thorium nitrate obtained from P. de Haen gave, when electrolysed, a deposit of lead peroxide on the anode. This deposit was radio-active, and its activity decayed at the normal rate of the excited activity due to thorium. From solutions of pure thorium nitrate, no visible deposit was obtained on the anode, but it was, however, found to be radio-active. The activity decayed rapidly, falling to half value in about one hour. Some experiments were also made on the effect of adding metallic salts to thorium solutions and then electrolysing them. Anode and cathode deposits of the oxides or metals obtained in this way were found to be radio-active, but the activity fell to half value in a few minutes. The gases produced by electrolysis were radio-active, but this was due to the presence of the thorium emanation. The explanation of the results obtained by Pegram and von Lerch will be considered later in [section 207]. It will be shown that the active deposit of thorium contains two distinct substances which have different rates of transformation.
187. Effect of temperature. The activity of a platinum wire which has been exposed in the presence of the thorium emanation is almost completely lost by heating the wire to a white heat. Miss F. Gates[[284]] found that the activity was not destroyed by the intense heat, but manifested itself on neighbouring bodies. When the active wire was heated electrically in a closed cylinder, the activity was transferred from the wire to the interior surface of the cylinder in unaltered amount. The rate of decay of the activity was not altered by the process. By blowing a current of air through the cylinder during the heating, a part of the active matter was removed from the cylinder. Similar results were found for the excited activity due to radium.
F. von Lerch (loc. cit.) determined the amount of activity removed at different temperatures. The results are shown in the following table for a platinum wire excited by the thorium emanation[[285]].
| Temperature | Percentage of activity removed | |
| Heated 2 minutes | 800° C. | 0 |
| then „ ½ minute more | 1020° C. | 16 |
| „ „ ½ „ „ | 1260° C. | 52 |
| „ „ ½ „ „ | 1460° C. | 99 |