Work on glaciers alerted Tyndall to the research of de Saussure into the heating effect of sunlight, and the concept of Fourier, developed by Pouillet and William Hopkins; that heat from the sun penetrates the atmosphere more easily than "obscure heat" (infrared) "terrestrial radiation" from the warmed Earth, causing what we now call the greenhouse effect. In the spring of 1859, Tyndall began research into how thermal radiation, both visible and obscure, affects different gases and aerosols. He developed differential absorption spectroscopy using the electro-magnetic thermopile devised by Melloni. Tyndall began intensive experiments on 9 May 1859, at first without significant results,