He most notably delivered a lecture entitled Observations on light and colours to the Medical Society of Edinburgh in 1752, in which he described what has been seen as the first flame test. In it he described how he had used a prism to observe a flame coloured by various salts. He reported that a yellow line was always seen at the same place in the spectrum; this was derived from the sodium which was present as an impurity in all his salts. Because of this, he is sometimes described as the father of flame emission spectroscopy, though he did not identify the source of the line, or propose his experiment as a method of analysis.