Critical to this resolution were improved methods of dating, which incorporated advances in mass spectrometry, sampling and laser heating. Even the Greeks and Romans realized that layers of sediment in rock signified old age. More important research was published in Rutherford wrote, I came into the room, which was half dark, and presently spotted Lord Kelvin in the audience and realized that I was in trouble at the last part of my speech dealing with the age of the Earth, where my views conflicted with his. They can then calculate, using the half-life of each parent, how long it took to create the differences between the amount of present-day lead and primordial lead for each of the three isotopes. In , he suggested that lead was the final stable product of the decay of radium.