The great chemist, willing to risk his life to stop the bloodshed. This page of the guide to Steelrising lists the locations of the Echoes tied to Antoine Lavoisier.
Antoine Lavoisier is a famous chemist and an important figure in Steelrising. He had decided to meet with the king's brother, hoping he can bring an end to the slaughter of innocents. There are seven Echoes tied to him total.
One last hope
This collectible is located inside the Louvre, in the Tuileries area. You'll find it in the Bailly Observatory, on the museum's top floor. You'll visit it during the main storyline.
In the lion's den
During The Shadows of Luxembourg quest, you will reach the Luxembourg Gardens location. While exploring it, you will eventually come across a gazebo - this is where the Echoe you are looking for is.
Daggers and venom
The opportunity to get this Echoe will also come during the previously mentioned quest (The Shadows of Luxembourg) and the journey through the Luxembourg Gardens. You can find it at the end of the area, before you enter the palace.
Opposite the bodies of the courtiers, you will find a hole in the railings. Jump down and follow the path. At the end of it you will find the missing collectible.