344 History of Luminescence 



responsible for phosphorescence, (1) tightly bound water and (2; 

 that present between the molecules. Light emission is connected 

 with the bound water, which may be excited by strong stimulation, 

 i. e., by high temperature (incandescence) or at low temperatures, 

 by collision, by electricity or by illumination. This type of lumi- 

 nescence ceases when the excitation ceases. The more easily excited 

 phosphorescence, also connected with bound water, which lasts for 

 some time after the exciting light is cut off, is a proof " that the 

 phosphoric fluid is easily concentrated and does not immediately 

 return to the resting condition." 



Naturally, Dessaignes had a difficult time fitting a theory to the 

 mass of observed facts which came from his experiments. This led 

 to changing ideas and inconsistencies in his statements. Briefly 

 stated, his theory appears to be fundamentally based on the belief 

 that only substances containing bound water can luminesce and 

 that this water takes up the phosphoric fluid (light substance) which 

 is nothing else than electricity. Each excitation acts like a shock 

 and sets the light substance free. Although Dessaignes' explanations 

 may be unacceptable, he (1810) appears to have been the first *^ to 

 expose phosphors to an electric discharge in a vacuum, the primary 

 step in the development of a fluorescent lamp. E. Becquerel was to 

 carry out similar but much more extensive studies in 1839. 



Heinrich relied on acid to explain phosphorescence. All lumi- 

 nescent substances contained an acid with which the light sub- 

 stance was associated. Therefore, he prepared his phosphors by 

 treating minerals with acid and heating. The acid was supposed to 

 be set free by decomposition as a result of illumination or warming 

 (thermoluminescence) and some of its associated light substance 

 became visible as luminescence. Naturally Heinrich encountered 

 the same difficulties with acid as Dessaignes had with water, and 

 endeavored to bolster his views with dubious chemical hypotheses. 

 For example, because some diamonds would phosphoresce he had 

 to assume they contained carbonic acid. Since ice phosphoresced 

 on illumination, he alleged the ice had more acid principle than 

 water. Heinrich also noted the marked action of the electric dis- 

 charge in exciting phosphorescence of minerals but, in opposition to 

 Dessaignes, correctly attributed it to the light and not to heat or 

 electricity.*^ His reasons were that glass between spark and phos- 

 phor had little effect in reducing the phosphorescence and that phos- 



*' Dessaignes, Jour, de physique 71: 67-70, 1810. 



"P. Heinrich, Die Phosphorescenz der Korper, Abt. I: 85 ff., 1811. Later (Abt. II: 

 238 ff., 1812) he wrote as if electricity were involved in restoration of the ability to 

 luminesce of calcite, fluorite, and heavy spar, when destroyed by heat. 



